diff --git a/packages/flutter_tools/lib/src/flutter_manifest.dart b/packages/flutter_tools/lib/src/flutter_manifest.dart
index fa30fe7a12e..67c38c68366 100644
--- a/packages/flutter_tools/lib/src/flutter_manifest.dart
+++ b/packages/flutter_tools/lib/src/flutter_manifest.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:meta/meta.dart';
 import 'package:pub_semver/pub_semver.dart';
 import 'package:yaml/yaml.dart';
@@ -21,20 +19,15 @@ const Set<String> _kValidPluginPlatforms = <String>{
 
 /// A wrapper around the `flutter` section in the `pubspec.yaml` file.
 class FlutterManifest {
-  FlutterManifest._(this._logger);
+  FlutterManifest._({required Logger logger}) : _logger = logger;
 
   /// Returns an empty manifest.
-  factory FlutterManifest.empty({ @required Logger logger }) {
-    final FlutterManifest manifest = FlutterManifest._(logger);
-    manifest._descriptor = const <String, dynamic>{};
-    manifest._flutterDescriptor = const <String, dynamic>{};
-    return manifest;
-  }
+  factory FlutterManifest.empty({ required Logger logger }) = FlutterManifest._;
 
   /// Returns null on invalid manifest. Returns empty manifest on missing file.
-  static FlutterManifest createFromPath(String path, {
-    @required FileSystem fileSystem,
-    @required Logger logger,
+  static FlutterManifest? createFromPath(String path, {
+    required FileSystem fileSystem,
+    required Logger logger,
   }) {
     if (path == null || !fileSystem.isFileSync(path)) {
       return _createFromYaml(null, logger);
@@ -45,28 +38,24 @@ class FlutterManifest {
 
   /// Returns null on missing or invalid manifest.
   @visibleForTesting
-  static FlutterManifest createFromString(String manifest, { @required Logger logger }) {
+  static FlutterManifest? createFromString(String manifest, { required Logger logger }) {
     return _createFromYaml(manifest != null ? loadYaml(manifest) : null, logger);
   }
 
-  static FlutterManifest _createFromYaml(dynamic yamlDocument, Logger logger) {
+  static FlutterManifest? _createFromYaml(Object? yamlDocument, Logger logger) {
     if (yamlDocument != null && !_validate(yamlDocument, logger)) {
       return null;
     }
 
-    final FlutterManifest pubspec = FlutterManifest._(logger);
-    final Map<dynamic, dynamic> yamlMap = yamlDocument as YamlMap;
+    final FlutterManifest pubspec = FlutterManifest._(logger: logger);
+    final Map<Object?, Object?>? yamlMap = yamlDocument as YamlMap?;
     if (yamlMap != null) {
-      pubspec._descriptor = yamlMap.cast<String, dynamic>();
-    } else {
-      pubspec._descriptor = <String, dynamic>{};
+      pubspec._descriptor = yamlMap.cast<String, Object?>();
     }
 
-    final Map<dynamic, dynamic> flutterMap = pubspec._descriptor['flutter'] as Map<dynamic, dynamic>;
+    final Map<Object?, Object?>? flutterMap = pubspec._descriptor['flutter'] as Map<Object?, Object?>?;
     if (flutterMap != null) {
-      pubspec._flutterDescriptor = flutterMap.cast<String, dynamic>();
-    } else {
-      pubspec._flutterDescriptor = <String, dynamic>{};
+      pubspec._flutterDescriptor = flutterMap.cast<String, Object?>();
     }
 
     return pubspec;
@@ -75,29 +64,29 @@ class FlutterManifest {
   final Logger _logger;
 
   /// A map representation of the entire `pubspec.yaml` file.
-  Map<String, dynamic> _descriptor;
+  Map<String, Object?> _descriptor = <String, Object?>{};
 
   /// A map representation of the `flutter` section in the `pubspec.yaml` file.
-  Map<String, dynamic> _flutterDescriptor;
+  Map<String, Object?> _flutterDescriptor = <String, Object?>{};
 
   /// True if the `pubspec.yaml` file does not exist.
   bool get isEmpty => _descriptor.isEmpty;
 
   /// The string value of the top-level `name` property in the `pubspec.yaml` file.
-  String get appName => _descriptor['name'] as String ?? '';
+  String get appName => _descriptor['name'] as String? ?? '';
 
   // Flag to avoid printing multiple invalid version messages.
   bool _hasShowInvalidVersionMsg = false;
 
   /// The version String from the `pubspec.yaml` file.
   /// Can be null if it isn't set or has a wrong format.
-  String get appVersion {
-    final String verStr = _descriptor['version']?.toString();
+  String? get appVersion {
+    final String? verStr = _descriptor['version']?.toString();
     if (verStr == null) {
       return null;
     }
 
-    Version version;
+    Version? version;
     try {
       version = Version.parse(verStr);
     } on Exception {
@@ -111,18 +100,20 @@ class FlutterManifest {
 
   /// The build version name from the `pubspec.yaml` file.
   /// Can be null if version isn't set or has a wrong format.
-  String get buildName {
-    if (appVersion != null && appVersion.contains('+')) {
-      return appVersion.split('+')?.elementAt(0);
+  String? get buildName {
+    final String? version = appVersion;
+    if (version != null && version.contains('+')) {
+      return version.split('+').elementAt(0);
     }
-    return appVersion;
+    return version;
   }
 
   /// The build version number from the `pubspec.yaml` file.
   /// Can be null if version isn't set or has a wrong format.
-  String get buildNumber {
-    if (appVersion != null && appVersion.contains('+')) {
-      final String value = appVersion.split('+')?.elementAt(1);
+  String? get buildNumber {
+    final String? version = appVersion;
+    if (version != null && version.contains('+')) {
+      final String value = version.split('+').elementAt(1);
       return value;
     } else {
       return null;
@@ -130,15 +121,16 @@ class FlutterManifest {
   }
 
   bool get usesMaterialDesign {
-    return _flutterDescriptor['uses-material-design'] as bool ?? false;
+    return _flutterDescriptor['uses-material-design'] as bool? ?? false;
   }
 
   /// True if this Flutter module should use AndroidX dependencies.
   ///
   /// If false the deprecated Android Support library will be used.
   bool get usesAndroidX {
-    if (_flutterDescriptor.containsKey('module')) {
-      return _flutterDescriptor['module']['androidX'] as bool;
+    final Object? module = _flutterDescriptor['module'];
+    if (module is YamlMap) {
+      return module['androidX'] == true;
     }
     return false;
   }
@@ -155,9 +147,13 @@ class FlutterManifest {
   ///   licenses:
   ///     - assets/foo_license.txt
   /// ```
-  List<String> get additionalLicenses => _flutterDescriptor.containsKey('licenses')
-    ? (_flutterDescriptor['licenses'] as YamlList).map((dynamic element) => element.toString()).toList()
-    : <String>[];
+  List<String> get additionalLicenses {
+    final Object? licenses = _flutterDescriptor['licenses'];
+    if (licenses is YamlList) {
+      return licenses.map((Object? element) => element.toString()).toList();
+    }
+    return <String>[];
+  }
 
   /// True if this manifest declares a Flutter module project.
   ///
@@ -181,51 +177,60 @@ class FlutterManifest {
   /// Returns the Android package declared by this manifest in its
   /// module or plugin descriptor. Returns null, if there is no
   /// such declaration.
-  String get androidPackage {
+  String? get androidPackage {
     if (isModule) {
-      return _flutterDescriptor['module']['androidPackage'] as String;
+      final Object? module = _flutterDescriptor['module'];
+      if (module is YamlMap) {
+        return module['androidPackage'] as String?;
+      }
     }
-    if (supportedPlatforms == null) {
+    final Map<String, Object?>? platforms = supportedPlatforms;
+    if (platforms == null) {
       // Pre-multi-platform plugin format
       if (isPlugin) {
-        final YamlMap plugin = _flutterDescriptor['plugin'] as YamlMap;
-        return plugin['androidPackage'] as String;
+        final YamlMap? plugin = _flutterDescriptor['plugin'] as YamlMap?;
+        return plugin?['androidPackage'] as String?;
       }
       return null;
     }
-    if (supportedPlatforms.containsKey('android')) {
-       return supportedPlatforms['android']['package'] as String;
+    if (platforms.containsKey('android')) {
+      final Object? android = platforms['android'];
+      if (android is YamlMap) {
+        return android['package'] as String?;
+      }
     }
     return null;
   }
 
   /// Returns the deferred components configuration if declared. Returns
   /// null if no deferred components are declared.
-  List<DeferredComponent> get deferredComponents => _deferredComponents ??= computeDeferredComponents();
-  List<DeferredComponent> _deferredComponents;
-  List<DeferredComponent> computeDeferredComponents() {
+  late final List<DeferredComponent>? deferredComponents = computeDeferredComponents();
+  List<DeferredComponent>? computeDeferredComponents() {
     if (!_flutterDescriptor.containsKey('deferred-components')) {
       return null;
     }
     final List<DeferredComponent> components = <DeferredComponent>[];
-    if (_flutterDescriptor['deferred-components'] == null) {
+    final Object? deferredComponents = _flutterDescriptor['deferred-components'];
+    if (deferredComponents is! YamlList) {
       return components;
     }
-    for (final dynamic componentData in _flutterDescriptor['deferred-components']) {
-      final YamlMap component = componentData as YamlMap;
+    for (final Object? component in deferredComponents) {
+      if (component is! YamlMap) {
+        _logger.printError('Expected deferred component manifest to be a map.');
+        continue;
+      }
       List<Uri> assetsUri = <Uri>[];
-      final List<dynamic> assets = component['assets'] as List<dynamic>;
+      final List<Object?>? assets = component['assets'] as List<Object?>?;
       if (assets == null) {
         assetsUri = const <Uri>[];
       } else {
-        for (final Object asset in assets) {
+        for (final Object? asset in assets) {
           if (asset is! String || asset == null || asset == '') {
             _logger.printError('Deferred component asset manifest contains a null or empty uri.');
             continue;
           }
-          final String stringAsset = asset as String;
           try {
-            assetsUri.add(Uri.parse(stringAsset));
+            assetsUri.add(Uri.parse(asset));
           } on FormatException {
             _logger.printError('Asset manifest contains invalid uri: $asset.');
           }
@@ -245,9 +250,12 @@ class FlutterManifest {
 
   /// Returns the iOS bundle identifier declared by this manifest in its
   /// module descriptor. Returns null if there is no such declaration.
-  String get iosBundleIdentifier {
+  String? get iosBundleIdentifier {
     if (isModule) {
-      return _flutterDescriptor['module']['iosBundleIdentifier'] as String;
+      final Object? module = _flutterDescriptor['module'];
+      if (module is YamlMap) {
+        return module['iosBundleIdentifier'] as String?;
+      }
     }
     return null;
   }
@@ -255,58 +263,56 @@ class FlutterManifest {
   /// Gets the supported platforms. This only supports the new `platforms` format.
   ///
   /// If the plugin uses the legacy pubspec format, this method returns null.
-  Map<String, dynamic> get supportedPlatforms {
+  Map<String, Object?>? get supportedPlatforms {
     if (isPlugin) {
-      final YamlMap plugin = _flutterDescriptor['plugin'] as YamlMap;
-      if (plugin.containsKey('platforms')) {
-        final YamlMap platformsMap = plugin['platforms'] as YamlMap;
-        return platformsMap.value.cast<String, dynamic>();
+      final YamlMap? plugin = _flutterDescriptor['plugin'] as YamlMap?;
+      if (plugin?.containsKey('platforms') == true) {
+        final YamlMap? platformsMap = plugin!['platforms'] as YamlMap?;
+        return platformsMap?.value.cast<String, Object?>();
       }
     }
     return null;
   }
 
   /// Like [supportedPlatforms], but only returns the valid platforms that are supported in flutter plugins.
-  Map<String, dynamic> get validSupportedPlatforms {
-    final Map<String, dynamic> allPlatforms = supportedPlatforms;
+  Map<String, Object?>? get validSupportedPlatforms {
+    final Map<String, Object?>? allPlatforms = supportedPlatforms;
     if (allPlatforms == null) {
       return null;
     }
-    final Map<String, dynamic> platforms = <String, dynamic>{}..addAll(supportedPlatforms);
-    platforms.removeWhere((String key, dynamic _) => !_kValidPluginPlatforms.contains(key));
+    final Map<String, Object?> platforms = <String, Object?>{}..addAll(allPlatforms);
+    platforms.removeWhere((String key, Object? _) => !_kValidPluginPlatforms.contains(key));
     if (platforms.isEmpty) {
       return null;
     }
     return platforms;
   }
 
-  List<Map<String, dynamic>> get fontsDescriptor {
+  List<Map<String, Object?>> get fontsDescriptor {
     return fonts.map((Font font) => font.descriptor).toList();
   }
 
-  List<Map<String, dynamic>> get _rawFontsDescriptor {
-    final List<dynamic> fontList = _flutterDescriptor['fonts'] as List<dynamic>;
+  List<Map<String, Object?>> get _rawFontsDescriptor {
+    final List<Object?>? fontList = _flutterDescriptor['fonts'] as List<Object?>?;
     return fontList == null
-        ? const <Map<String, dynamic>>[]
-        : fontList.map<Map<String, dynamic>>(castStringKeyedMap).toList();
+        ? const <Map<String, Object?>>[]
+        : fontList.map<Map<String, Object?>?>(castStringKeyedMap).whereType<Map<String, Object?>>().toList();
   }
 
-  List<Uri> get assets => _assets ??= _computeAssets();
-  List<Uri> _assets;
+  late final List<Uri> assets = _computeAssets();
   List<Uri> _computeAssets() {
-    final List<dynamic> assets = _flutterDescriptor['assets'] as List<dynamic>;
+    final List<Object?>? assets = _flutterDescriptor['assets'] as List<Object?>?;
     if (assets == null) {
       return const <Uri>[];
     }
     final List<Uri> results = <Uri>[];
-    for (final Object asset in assets) {
+    for (final Object? asset in assets) {
       if (asset is! String || asset == null || asset == '') {
         _logger.printError('Asset manifest contains a null or empty uri.');
         continue;
       }
-      final String stringAsset = asset as String;
       try {
-        results.add(Uri(pathSegments: stringAsset.split('/')));
+        results.add(Uri(pathSegments: asset.split('/')));
       } on FormatException {
         _logger.printError('Asset manifest contains invalid uri: $asset.');
       }
@@ -314,12 +320,7 @@ class FlutterManifest {
     return results;
   }
 
-  List<Font> _fonts;
-
-  List<Font> get fonts {
-    _fonts ??= _extractFonts();
-    return _fonts;
-  }
+  late final List<Font> fonts = _extractFonts();
 
   List<Font> _extractFonts() {
     if (!_flutterDescriptor.containsKey('fonts')) {
@@ -327,9 +328,9 @@ class FlutterManifest {
     }
 
     final List<Font> fonts = <Font>[];
-    for (final Map<String, dynamic> fontFamily in _rawFontsDescriptor) {
-      final YamlList fontFiles = fontFamily['fonts'] as YamlList;
-      final String familyName = fontFamily['family'] as String;
+    for (final Map<String, Object?> fontFamily in _rawFontsDescriptor) {
+      final YamlList? fontFiles = fontFamily['fonts'] as YamlList?;
+      final String? familyName = fontFamily['family'] as String?;
       if (familyName == null) {
         _logger.printError('Warning: Missing family name for font.', emphasis: true);
         continue;
@@ -340,8 +341,8 @@ class FlutterManifest {
       }
 
       final List<FontAsset> fontAssets = <FontAsset>[];
-      for (final Map<dynamic, dynamic> fontFile in fontFiles.cast<Map<dynamic, dynamic>>()) {
-        final String asset = fontFile['asset'] as String;
+      for (final Map<Object?, Object?> fontFile in fontFiles.cast<Map<Object?, Object?>>()) {
+        final String? asset = fontFile['asset'] as String?;
         if (asset == null) {
           _logger.printError('Warning: Missing asset in fonts for $familyName', emphasis: true);
           continue;
@@ -349,12 +350,12 @@ class FlutterManifest {
 
         fontAssets.add(FontAsset(
           Uri.parse(asset),
-          weight: fontFile['weight'] as int,
-          style: fontFile['style'] as String,
+          weight: fontFile['weight'] as int?,
+          style: fontFile['style'] as String?,
         ));
       }
       if (fontAssets.isNotEmpty) {
-        fonts.add(Font(fontFamily['family'] as String, fontAssets));
+        fonts.add(Font(familyName, fontAssets));
       }
     }
     return fonts;
@@ -367,17 +368,16 @@ class FlutterManifest {
   ///
   /// This allows generated source code to be imported using a package
   /// alias.
-  bool get generateSyntheticPackage => _generateSyntheticPackage ??= _computeGenerateSyntheticPackage();
-  bool _generateSyntheticPackage;
+  late final bool generateSyntheticPackage = _computeGenerateSyntheticPackage();
   bool _computeGenerateSyntheticPackage() {
     if (!_flutterDescriptor.containsKey('generate')) {
       return false;
     }
-    final Object value = _flutterDescriptor['generate'];
+    final Object? value = _flutterDescriptor['generate'];
     if (value is! bool) {
       return false;
     }
-    return value as bool;
+    return value;
   }
 }
 
@@ -390,10 +390,10 @@ class Font {
   final String familyName;
   final List<FontAsset> fontAssets;
 
-  Map<String, dynamic> get descriptor {
-    return <String, dynamic>{
+  Map<String, Object?> get descriptor {
+    return <String, Object?>{
       'family': familyName,
-      'fonts': fontAssets.map<Map<String, dynamic>>((FontAsset a) => a.descriptor).toList(),
+      'fonts': fontAssets.map<Map<String, Object?>>((FontAsset a) => a.descriptor).toList(),
     };
   }
 
@@ -406,11 +406,11 @@ class FontAsset {
     : assert(assetUri != null);
 
   final Uri assetUri;
-  final int weight;
-  final String style;
+  final int? weight;
+  final String? style;
 
-  Map<String, dynamic> get descriptor {
-    final Map<String, dynamic> descriptor = <String, dynamic>{};
+  Map<String, Object?> get descriptor {
+    final Map<String, Object?> descriptor = <String, Object?>{};
     if (weight != null) {
       descriptor['weight'] = weight;
     }
@@ -428,17 +428,17 @@ class FontAsset {
 }
 
 
-bool _validate(dynamic manifest, Logger logger) {
+bool _validate(Object? manifest, Logger logger) {
   final List<String> errors = <String>[];
   if (manifest is! YamlMap) {
     errors.add('Expected YAML map');
   } else {
-    for (final MapEntry<dynamic, dynamic> kvp in (manifest as YamlMap).entries) {
+    for (final MapEntry<Object?, Object?> kvp in manifest.entries) {
       if (kvp.key is! String) {
         errors.add('Expected YAML key to be a string, but got ${kvp.key}.');
         continue;
       }
-      switch (kvp.key as String) {
+      switch (kvp.key as String?) {
         case 'name':
           if (kvp.value is! String) {
             errors.add('Expected "${kvp.key}" to be a string, but got ${kvp.value}.');
@@ -451,7 +451,7 @@ bool _validate(dynamic manifest, Logger logger) {
           if (kvp.value is! YamlMap) {
             errors.add('Expected "${kvp.key}" section to be an object or null, but got ${kvp.value}.');
           } else {
-            _validateFlutter(kvp.value as YamlMap, errors);
+            _validateFlutter(kvp.value as YamlMap?, errors);
           }
           break;
         default:
@@ -470,62 +470,64 @@ bool _validate(dynamic manifest, Logger logger) {
   return true;
 }
 
-void _validateFlutter(YamlMap yaml, List<String> errors) {
+void _validateFlutter(YamlMap? yaml, List<String> errors) {
   if (yaml == null || yaml.entries == null) {
     return;
   }
-  for (final MapEntry<dynamic, dynamic> kvp in yaml.entries) {
-    if (kvp.key is! String) {
-      errors.add('Expected YAML key to be a string, but got ${kvp.key} (${kvp.value.runtimeType}).');
+  for (final MapEntry<Object?, Object?> kvp in yaml.entries) {
+    final Object? yamlKey = kvp.key;
+    final Object? yamlValue = kvp.value;
+    if (yamlKey is! String) {
+      errors.add('Expected YAML key to be a string, but got $yamlKey (${yamlValue.runtimeType}).');
       continue;
     }
-    switch (kvp.key as String) {
+    switch (yamlKey) {
       case 'uses-material-design':
-        if (kvp.value is! bool) {
-          errors.add('Expected "${kvp.key}" to be a bool, but got ${kvp.value} (${kvp.value.runtimeType}).');
+        if (yamlValue is! bool) {
+          errors.add('Expected "$yamlKey" to be a bool, but got $yamlValue (${yamlValue.runtimeType}).');
         }
         break;
       case 'assets':
-        if (kvp.value is! YamlList || kvp.value[0] is! String) {
-          errors.add('Expected "${kvp.key}" to be a list, but got ${kvp.value} (${kvp.value.runtimeType}).');
+        if (yamlValue is! YamlList || yamlValue[0] is! String) {
+          errors.add('Expected "$yamlKey" to be a list, but got $yamlValue (${yamlValue.runtimeType}).');
         }
         break;
       case 'fonts':
-        if (kvp.value is! YamlList || kvp.value[0] is! YamlMap) {
-          errors.add('Expected "${kvp.key}" to be a list, but got ${kvp.value} (${kvp.value.runtimeType}).');
+        if (yamlValue is! YamlList || yamlValue[0] is! YamlMap) {
+          errors.add('Expected "$yamlKey" to be a list, but got $yamlValue (${yamlValue.runtimeType}).');
         } else {
-          _validateFonts(kvp.value as YamlList, errors);
+          _validateFonts(yamlValue, errors);
         }
         break;
       case 'licenses':
-        final dynamic value = kvp.value;
-        if (value is YamlList) {
-          _validateListType<String>(value, errors, '"${kvp.key}"', 'files');
+        if (yamlValue is YamlList) {
+          _validateListType<String>(yamlValue, errors, '"$yamlKey"', 'files');
         } else {
-          errors.add('Expected "${kvp.key}" to be a list of files, but got $value (${value.runtimeType})');
+          errors.add('Expected "$yamlKey" to be a list of files, but got $yamlValue (${yamlValue.runtimeType})');
         }
         break;
       case 'module':
-        if (kvp.value is! YamlMap) {
-          errors.add('Expected "${kvp.key}" to be an object, but got ${kvp.value} (${kvp.value.runtimeType}).');
+        if (yamlValue is! YamlMap) {
+          errors.add('Expected "$yamlKey" to be an object, but got $yamlValue (${yamlValue.runtimeType}).');
+          break;
         }
 
-        if (kvp.value['androidX'] != null && kvp.value['androidX'] is! bool) {
+        if (yamlValue['androidX'] != null && yamlValue['androidX'] is! bool) {
           errors.add('The "androidX" value must be a bool if set.');
         }
-        if (kvp.value['androidPackage'] != null && kvp.value['androidPackage'] is! String) {
+        if (yamlValue['androidPackage'] != null && yamlValue['androidPackage'] is! String) {
           errors.add('The "androidPackage" value must be a string if set.');
         }
-        if (kvp.value['iosBundleIdentifier'] != null && kvp.value['iosBundleIdentifier'] is! String) {
+        if (yamlValue['iosBundleIdentifier'] != null && yamlValue['iosBundleIdentifier'] is! String) {
           errors.add('The "iosBundleIdentifier" section must be a string if set.');
         }
         break;
       case 'plugin':
-        if (kvp.value is! YamlMap || kvp.value == null) {
-          errors.add('Expected "${kvp.key}" to be an object, but got ${kvp.value} (${kvp.value.runtimeType}).');
+        if (yamlValue is! YamlMap || yamlValue == null) {
+          errors.add('Expected "$yamlKey" to be an object, but got $yamlValue (${yamlValue.runtimeType}).');
           break;
         }
-        final List<String> pluginErrors = Plugin.validatePluginYaml(kvp.value as YamlMap);
+        final List<String> pluginErrors = Plugin.validatePluginYaml(yamlValue);
         errors.addAll(pluginErrors);
         break;
       case 'generate':
@@ -534,7 +536,7 @@ void _validateFlutter(YamlMap yaml, List<String> errors) {
         _validateDeferredComponents(kvp, errors);
         break;
       default:
-        errors.add('Unexpected child "${kvp.key}" found under "flutter".');
+        errors.add('Unexpected child "$yamlKey" found under "flutter".');
         break;
     }
   }
@@ -548,30 +550,34 @@ void _validateListType<T>(YamlList yamlList, List<String> errors, String context
   }
 }
 
-void _validateDeferredComponents(MapEntry<dynamic, dynamic> kvp, List<String> errors) {
-  if (kvp.value != null && (kvp.value is! YamlList || kvp.value[0] is! YamlMap)) {
-    errors.add('Expected "${kvp.key}" to be a list, but got ${kvp.value} (${kvp.value.runtimeType}).');
-  } else if (kvp.value != null) {
-    for (int i = 0; i < (kvp.value as YamlList).length; i++) {
-      if (kvp.value[i] is! YamlMap) {
-        errors.add('Expected the $i element in "${kvp.key}" to be a map, but got ${kvp.value[i]} (${kvp.value[i].runtimeType}).');
+void _validateDeferredComponents(MapEntry<Object?, Object?> kvp, List<String> errors) {
+  final Object? yamlList = kvp.value;
+  if (yamlList != null && (yamlList is! YamlList || yamlList[0] is! YamlMap)) {
+    errors.add('Expected "${kvp.key}" to be a list, but got $yamlList (${yamlList.runtimeType}).');
+  } else if (yamlList is YamlList) {
+    for (int i = 0; i < yamlList.length; i++) {
+      final Object? valueMap = yamlList[i];
+      if (valueMap is! YamlMap) {
+        errors.add('Expected the $i element in "${kvp.key}" to be a map, but got ${yamlList[i]} (${yamlList[i].runtimeType}).');
         continue;
       }
-      if (!(kvp.value[i] as YamlMap).containsKey('name') || kvp.value[i]['name'] is! String) {
+      if (!valueMap.containsKey('name') || valueMap['name'] is! String) {
         errors.add('Expected the $i element in "${kvp.key}" to have required key "name" of type String');
       }
-      if ((kvp.value[i] as YamlMap).containsKey('libraries')) {
-        if (kvp.value[i]['libraries'] is! YamlList) {
-          errors.add('Expected "libraries" key in the $i element of "${kvp.key}" to be a list, but got ${kvp.value[i]['libraries']} (${kvp.value[i]['libraries'].runtimeType}).');
+      if (valueMap.containsKey('libraries')) {
+        final Object? libraries = valueMap['libraries'];
+        if (libraries is! YamlList) {
+          errors.add('Expected "libraries" key in the $i element of "${kvp.key}" to be a list, but got $libraries (${libraries.runtimeType}).');
         } else {
-          _validateListType<String>(kvp.value[i]['libraries'] as YamlList, errors, '"libraries" key in the $i element of "${kvp.key}"', 'dart library Strings');
+          _validateListType<String>(libraries, errors, '"libraries" key in the $i element of "${kvp.key}"', 'dart library Strings');
         }
       }
-      if ((kvp.value[i] as YamlMap).containsKey('assets')) {
-        if (kvp.value[i]['assets'] is! YamlList) {
-          errors.add('Expected "assets" key in the $i element of "${kvp.key}" to be a list, but got ${kvp.value[i]['assets']} (${kvp.value[i]['assets'].runtimeType}).');
+      if (valueMap.containsKey('assets')) {
+        final Object? assets = valueMap['assets'];
+        if (assets is! YamlList) {
+          errors.add('Expected "assets" key in the $i element of "${kvp.key}" to be a list, but got $assets (${assets.runtimeType}).');
         } else {
-          _validateListType<String>(kvp.value[i]['assets'] as YamlList, errors, '"assets" key in the $i element of "${kvp.key}"', 'file paths');
+          _validateListType<String>(assets, errors, '"assets" key in the $i element of "${kvp.key}"', 'file paths');
         }
       }
     }
@@ -585,13 +591,12 @@ void _validateFonts(YamlList fonts, List<String> errors) {
   const Set<int> fontWeights = <int>{
     100, 200, 300, 400, 500, 600, 700, 800, 900,
   };
-  for (final dynamic fontListEntry in fonts) {
-    if (fontListEntry is! YamlMap) {
-      errors.add('Unexpected child "$fontListEntry" found under "fonts". Expected a map.');
+  for (final Object? fontMap in fonts) {
+    if (fontMap is! YamlMap) {
+      errors.add('Unexpected child "$fontMap" found under "fonts". Expected a map.');
       continue;
     }
-    final YamlMap fontMap = fontListEntry as YamlMap;
-    for (final dynamic key in fontMap.keys.where((dynamic key) => key != 'family' && key != 'fonts')) {
+    for (final Object? key in fontMap.keys.where((Object? key) => key != 'family' && key != 'fonts')) {
       errors.add('Unexpected child "$key" found under "fonts".');
     }
     if (fontMap['family'] != null && fontMap['family'] is! String) {
@@ -603,17 +608,17 @@ void _validateFonts(YamlList fonts, List<String> errors) {
       errors.add('Expected "fonts" to either be null or a list.');
       continue;
     }
-    for (final dynamic fontListItem in fontMap['fonts']) {
-      if (fontListItem is! YamlMap) {
+    for (final Object? fontMapList in fontMap['fonts']) {
+      if (fontMapList is! YamlMap) {
         errors.add('Expected "fonts" to be a list of maps.');
         continue;
       }
-      final YamlMap fontMapList = fontListItem as YamlMap;
-      for (final MapEntry<dynamic, dynamic> kvp in fontMapList.entries) {
-        if (kvp.key is! String) {
-          errors.add('Expected "${kvp.key}" under "fonts" to be a string.');
+      for (final MapEntry<Object?, Object?> kvp in fontMapList.entries) {
+        final Object? fontKey = kvp.key;
+        if (fontKey is! String) {
+          errors.add('Expected "$fontKey" under "fonts" to be a string.');
         }
-        switch(kvp.key as String) {
+        switch(fontKey) {
           case 'asset':
             if (kvp.value is! String) {
               errors.add('Expected font asset ${kvp.value} ((${kvp.value.runtimeType})) to be a string.');
@@ -630,7 +635,7 @@ void _validateFonts(YamlList fonts, List<String> errors) {
             }
             break;
           default:
-            errors.add('Unexpected key ${kvp.key} ((${kvp.value.runtimeType})) under font.');
+            errors.add('Unexpected key $fontKey ((${kvp.value.runtimeType})) under font.');
             break;
         }
       }
diff --git a/packages/flutter_tools/test/general.shard/flutter_manifest_test.dart b/packages/flutter_tools/test/general.shard/flutter_manifest_test.dart
index eb2a0712b46..709ab208309 100644
--- a/packages/flutter_tools/test/general.shard/flutter_manifest_test.dart
+++ b/packages/flutter_tools/test/general.shard/flutter_manifest_test.dart
@@ -2,17 +2,15 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:file/file.dart';
 import 'package:file/memory.dart';
+import 'package:flutter_tools/src/base/deferred_component.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/logger.dart';
 import 'package:flutter_tools/src/cache.dart';
 import 'package:flutter_tools/src/flutter_manifest.dart';
 
 import '../src/common.dart';
-import '../src/context.dart';
 
 void main() {
   setUpAll(() {
@@ -24,7 +22,7 @@ void main() {
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       '',
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.isEmpty, true);
     expect(flutterManifest.appName, '');
@@ -56,7 +54,7 @@ dependencies:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest, isNotNull);
     expect(flutterManifest.isEmpty, false);
@@ -80,7 +78,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.usesMaterialDesign, true);
   });
@@ -98,7 +96,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.generateSyntheticPackage, true);
   });
@@ -116,7 +114,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.generateSyntheticPackage, false);
   });
@@ -134,7 +132,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.generateSyntheticPackage, false);
   });
@@ -155,7 +153,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.assets, <Uri>[
       Uri.parse('a/foo'),
@@ -180,7 +178,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.fonts, hasLength(1));
     expect(flutterManifest.fonts.single, matchesFont(
@@ -217,7 +215,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.fonts, hasLength(1));
     expect(flutterManifest.fonts.single, matchesFont(
@@ -257,7 +255,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.fonts, hasLength(1));
     expect(flutterManifest.fonts.single, matchesFont(
@@ -303,7 +301,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.fonts, hasLength(2));
     expect(flutterManifest.fonts, containsAll(<Matcher>[
@@ -365,7 +363,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.fontsDescriptor, <Object>[
       <String, Object>{
@@ -411,7 +409,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.fonts, hasLength(1));
     expect(flutterManifest.fonts, containsAll(<Matcher>[
@@ -454,7 +452,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.fonts, hasLength(1));
     expect(flutterManifest.fonts, containsAll(<Matcher>[
@@ -493,7 +491,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.fontsDescriptor, isEmpty);
     expect(flutterManifest.fonts, isEmpty);
@@ -511,7 +509,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.isEmpty, false);
     expect(flutterManifest.isModule, false);
@@ -532,7 +530,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.isModule, true);
     expect(flutterManifest.androidPackage, 'com.example');
@@ -550,7 +548,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.isPlugin, true);
     expect(flutterManifest.androidPackage, 'com.example');
@@ -571,7 +569,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.isPlugin, true);
     expect(flutterManifest.androidPackage, 'com.example');
@@ -591,20 +589,20 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.isPlugin, true);
     expect(flutterManifest.androidPackage, isNull);
   });
 
-  testUsingContext('FlutterManifest handles an invalid plugin declaration', () {
+  testWithoutContext('FlutterManifest handles an invalid plugin declaration', () {
     const String manifest = '''
 name: test
 flutter:
     plugin:
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -624,7 +622,7 @@ dependencies:
 flutter:
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -646,7 +644,7 @@ dependencies:
 flutter:
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -668,7 +666,7 @@ dependencies:
 flutter:
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -690,7 +688,7 @@ dependencies:
 flutter:
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -712,7 +710,7 @@ dependencies:
 flutter:
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -733,7 +731,7 @@ dependencies:
 flutter:
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -759,7 +757,7 @@ flutter:
         -asset: a/bar
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -783,7 +781,7 @@ flutter:
         - asset
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -807,7 +805,7 @@ flutter:
       -asset: a/bar
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -832,7 +830,7 @@ flutter:
     - string
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -857,7 +855,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
     final List<Uri> assets = flutterManifest.assets;
 
     expect(logger.errorText, contains('Asset manifest contains a null or empty uri.'));
@@ -881,7 +879,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
     final List<Uri> assets = flutterManifest.assets;
 
     expect(assets, hasLength(3));
@@ -903,7 +901,7 @@ flutter:
   - uses-material-design: true
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -932,7 +930,7 @@ flutter:
        'pubspec.yaml',
       fileSystem: fileSystem,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.isEmpty, false);
   });
@@ -953,7 +951,7 @@ flutter:
       'pubspec.yaml',
       fileSystem: fileSystem,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.isEmpty, false);
   });
@@ -969,7 +967,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.isPlugin, true);
     expect(flutterManifest.supportedPlatforms, null);
@@ -988,7 +986,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.isPlugin, true);
     expect(flutterManifest.validSupportedPlatforms, null);
@@ -1009,12 +1007,13 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.isPlugin, true);
-    expect(flutterManifest.validSupportedPlatforms['ios'],
+    final Map<String, dynamic> validSupportedPlatforms = flutterManifest.validSupportedPlatforms!;
+    expect(validSupportedPlatforms['ios'],
                               <String, dynamic>{'pluginClass': 'SomeClass'});
-    expect(flutterManifest.validSupportedPlatforms['some_platform'],
+    expect(validSupportedPlatforms['some_platform'],
                               isNull);
   });
 
@@ -1034,12 +1033,13 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.isPlugin, true);
-    expect(flutterManifest.supportedPlatforms['ios'],
+    final Map<String, dynamic> validSupportedPlatforms = flutterManifest.validSupportedPlatforms!;
+    expect(validSupportedPlatforms['ios'],
                               <String, dynamic>{'pluginClass': 'SomeClass'});
-    expect(flutterManifest.supportedPlatforms['android'],
+    expect(validSupportedPlatforms['android'],
                               <String, dynamic>{'pluginClass': 'SomeClass',
                                                 'package': 'com.example'});
   });
@@ -1054,7 +1054,7 @@ flutter:
         - android
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -1076,7 +1076,7 @@ flutter:
       pluginClass: SomeClass
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -1100,7 +1100,7 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest.additionalLicenses, <String>['foo.txt']);
   });
@@ -1115,7 +1115,7 @@ flutter:
   licenses: foo.txt
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -1136,7 +1136,7 @@ flutter:
     - bar: fizz
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -1163,15 +1163,16 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest, isNotNull);
-    expect(flutterManifest.deferredComponents.length, 1);
-    expect(flutterManifest.deferredComponents[0].name, 'component1');
-    expect(flutterManifest.deferredComponents[0].libraries.length, 1);
-    expect(flutterManifest.deferredComponents[0].libraries[0], 'lib1');
-    expect(flutterManifest.deferredComponents[0].assets.length, 1);
-    expect(flutterManifest.deferredComponents[0].assets[0].path, 'path/to/asset.jpg');
+    final List<DeferredComponent> deferredComponents = flutterManifest.deferredComponents!;
+    expect(deferredComponents.length, 1);
+    expect(deferredComponents[0].name, 'component1');
+    expect(deferredComponents[0].libraries.length, 1);
+    expect(deferredComponents[0].libraries[0], 'lib1');
+    expect(deferredComponents[0].assets.length, 1);
+    expect(deferredComponents[0].assets[0].path, 'path/to/asset.jpg');
   });
 
   testWithoutContext('FlutterManifest parses multiple deferred components', () async {
@@ -1198,22 +1199,23 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest, isNotNull);
-    expect(flutterManifest.deferredComponents.length, 2);
-    expect(flutterManifest.deferredComponents[0].name, 'component1');
-    expect(flutterManifest.deferredComponents[0].libraries.length, 1);
-    expect(flutterManifest.deferredComponents[0].libraries[0], 'lib1');
-    expect(flutterManifest.deferredComponents[0].assets.length, 1);
-    expect(flutterManifest.deferredComponents[0].assets[0].path, 'path/to/asset.jpg');
+    final List<DeferredComponent> deferredComponents = flutterManifest.deferredComponents!;
+    expect(deferredComponents.length, 2);
+    expect(deferredComponents[0].name, 'component1');
+    expect(deferredComponents[0].libraries.length, 1);
+    expect(deferredComponents[0].libraries[0], 'lib1');
+    expect(deferredComponents[0].assets.length, 1);
+    expect(deferredComponents[0].assets[0].path, 'path/to/asset.jpg');
 
-    expect(flutterManifest.deferredComponents[1].name, 'component2');
-    expect(flutterManifest.deferredComponents[1].libraries.length, 2);
-    expect(flutterManifest.deferredComponents[1].libraries[0], 'lib2');
-    expect(flutterManifest.deferredComponents[1].libraries[1], 'lib3');
-    expect(flutterManifest.deferredComponents[1].assets.length, 1);
-    expect(flutterManifest.deferredComponents[1].assets[0].path, 'path/to/asset2.jpg');
+    expect(deferredComponents[1].name, 'component2');
+    expect(deferredComponents[1].libraries.length, 2);
+    expect(deferredComponents[1].libraries[0], 'lib2');
+    expect(deferredComponents[1].libraries[1], 'lib3');
+    expect(deferredComponents[1].assets.length, 1);
+    expect(deferredComponents[1].assets[0].path, 'path/to/asset2.jpg');
   });
 
   testWithoutContext('FlutterManifest parses empty deferred components', () async {
@@ -1229,10 +1231,10 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest, isNotNull);
-    expect(flutterManifest.deferredComponents.length, 0);
+    expect(flutterManifest.deferredComponents!.length, 0);
   });
 
   testWithoutContext('FlutterManifest deferred component requires name', () async {
@@ -1247,7 +1249,7 @@ flutter:
         - lib1
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -1266,7 +1268,7 @@ flutter:
   deferred-components: blah
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -1287,7 +1289,7 @@ flutter:
       libraries: blah
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -1309,7 +1311,7 @@ flutter:
         - not-a-string:
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -1331,7 +1333,7 @@ flutter:
         - not-a-string:
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -1355,7 +1357,7 @@ flutter:
           - woo
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -1382,7 +1384,7 @@ flutter:
           - woo
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -1403,7 +1405,7 @@ flutter:
       assets: blah
 ''';
     final BufferLogger logger = BufferLogger.test();
-    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
     );
@@ -1430,23 +1432,24 @@ flutter:
     final FlutterManifest flutterManifest = FlutterManifest.createFromString(
       manifest,
       logger: logger,
-    );
+    )!;
 
     expect(flutterManifest, isNotNull);
-    expect(flutterManifest.deferredComponents.length, 1);
-    expect(flutterManifest.deferredComponents[0].name, 'component1');
-    expect(flutterManifest.deferredComponents[0].libraries.length, 0);
-    expect(flutterManifest.deferredComponents[0].assets.length, 3);
-    expect(flutterManifest.deferredComponents[0].assets[0].path, 'path/to/asset1.jpg');
-    expect(flutterManifest.deferredComponents[0].assets[1].path, 'path/to/asset2.jpg');
-    expect(flutterManifest.deferredComponents[0].assets[2].path, 'path/to/asset3.jpg');
+    final List<DeferredComponent> deferredComponents = flutterManifest.deferredComponents!;
+    expect(deferredComponents.length, 1);
+    expect(deferredComponents[0].name, 'component1');
+    expect(deferredComponents[0].libraries.length, 0);
+    expect(deferredComponents[0].assets.length, 3);
+    expect(deferredComponents[0].assets[0].path, 'path/to/asset1.jpg');
+    expect(deferredComponents[0].assets[1].path, 'path/to/asset2.jpg');
+    expect(deferredComponents[0].assets[2].path, 'path/to/asset3.jpg');
   });
 }
 
 Matcher matchesManifest({
-  String appVersion,
-  String buildName,
-  String buildNumber,
+  String? appVersion,
+  String? buildName,
+  String? buildNumber,
 }) {
   return isA<FlutterManifest>()
     .having((FlutterManifest manifest) => manifest.appVersion, 'appVersion', appVersion)
@@ -1455,9 +1458,9 @@ Matcher matchesManifest({
 }
 
 Matcher matchesFontAsset({
-  Uri assetUri,
-  int weight,
-  String style,
+  required Uri assetUri,
+  int? weight,
+  String? style,
 }) {
   return isA<FontAsset>()
     .having((FontAsset fontAsset) => fontAsset.assetUri, 'assetUri', assetUri)
@@ -1466,9 +1469,9 @@ Matcher matchesFontAsset({
 }
 
 Matcher matchesFont({
-  Map<String, Object> descriptor,
-  String familyName,
-  List<Matcher> fontAssets,
+  required Map<String, Object> descriptor,
+  required String familyName,
+  required List<Matcher> fontAssets,
 }) {
   return isA<Font>()
     .having((Font font) => font.descriptor, 'descriptor', descriptor)