diff --git a/packages/flutter_tools/lib/src/build_system/targets/localizations.dart b/packages/flutter_tools/lib/src/build_system/targets/localizations.dart
index 6767ca308ed..05c0e7fd382 100644
--- a/packages/flutter_tools/lib/src/build_system/targets/localizations.dart
+++ b/packages/flutter_tools/lib/src/build_system/targets/localizations.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 '../../base/file_system.dart';
 import '../../convert.dart';
 import '../../localizations/gen_l10n.dart';
@@ -69,15 +67,19 @@ class GenerateLocalizationsTarget extends Target {
     final Map<String, Object> dependencies = json.decode(
       environment.buildDir.childFile(_kDependenciesFileName).readAsStringSync()
     ) as Map<String, Object>;
+    final List<Object?>? inputs = dependencies['inputs'] as List<Object?>?;
+    final List<Object?>? outputs = dependencies['outputs'] as List<Object?>?;
     final Depfile depfile = Depfile(
       <File>[
         configFile,
-        for (dynamic inputFile in dependencies['inputs'] as List<dynamic>)
-          environment.fileSystem.file(inputFile)
+        if (inputs != null)
+          for (Object inputFile in inputs.whereType<Object>())
+            environment.fileSystem.file(inputFile)
       ],
       <File>[
-        for (dynamic outputFile in dependencies['outputs'] as List<dynamic>)
-          environment.fileSystem.file(outputFile)
+        if (outputs != null)
+          for (Object outputFile in outputs.whereType<Object>())
+            environment.fileSystem.file(outputFile)
       ],
     );
     depfileService.writeToFile(
diff --git a/packages/flutter_tools/lib/src/dart/generate_synthetic_packages.dart b/packages/flutter_tools/lib/src/dart/generate_synthetic_packages.dart
index 66ca9b38ef2..039add90642 100644
--- a/packages/flutter_tools/lib/src/dart/generate_synthetic_packages.dart
+++ b/packages/flutter_tools/lib/src/dart/generate_synthetic_packages.dart
@@ -2,9 +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:yaml/yaml.dart';
 
 import '../base/common.dart';
@@ -14,8 +11,8 @@ import '../build_system/build_system.dart';
 import '../build_system/targets/localizations.dart';
 
 Future<void> generateLocalizationsSyntheticPackage({
-  @required Environment environment,
-  @required BuildSystem buildSystem,
+  required Environment environment,
+  required BuildSystem buildSystem,
 }) async {
   assert(environment != null);
   assert(buildSystem != null);
@@ -42,7 +39,7 @@ Future<void> generateLocalizationsSyntheticPackage({
   // it.
   if (yamlNode.value != null) {
     final YamlMap yamlMap = yamlNode as YamlMap;
-    final Object value = yamlMap['synthetic-package'];
+    final Object? value = yamlMap['synthetic-package'];
     if (value is! bool && value != null) {
       throwToolExit(
         'Expected "synthetic-package" to have a bool value, '
@@ -52,8 +49,8 @@ Future<void> generateLocalizationsSyntheticPackage({
 
     // Generate gen_l10n synthetic package only if synthetic-package: true or
     // synthetic-package is null.
-    final bool isSyntheticL10nPackage = value as bool ?? true;
-    if (!isSyntheticL10nPackage) {
+    final bool? isSyntheticL10nPackage = value as bool?;
+    if (isSyntheticL10nPackage == false) {
       return;
     }
   }
@@ -70,7 +67,7 @@ Future<void> generateLocalizationsSyntheticPackage({
     throwToolExit(
       'Generating synthetic localizations package failed with ${result.exceptions.length} ${pluralize('error', result.exceptions.length)}:'
       '\n\n'
-      '${result.exceptions.values.map<Object>((ExceptionMeasurement e) => e.exception).join('\n\n')}',
+      '${result.exceptions.values.map<Object?>((ExceptionMeasurement e) => e.exception).join('\n\n')}',
     );
   }
 }
diff --git a/packages/flutter_tools/test/general.shard/build_system/targets/localizations_test.dart b/packages/flutter_tools/test/general.shard/build_system/targets/localizations_test.dart
index 64e053e47d1..4e07a525c85 100644
--- a/packages/flutter_tools/test/general.shard/build_system/targets/localizations_test.dart
+++ b/packages/flutter_tools/test/general.shard/build_system/targets/localizations_test.dart
@@ -2,9 +2,8 @@
 // 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/memory.dart';
+import 'package:flutter_tools/src/artifacts.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/logger.dart';
 import 'package:flutter_tools/src/build_system/build_system.dart';
@@ -19,7 +18,7 @@ void main() {
     final FileSystem fileSystem = MemoryFileSystem.test();
     final Environment environment = Environment.test(
       fileSystem.currentDirectory,
-      artifacts: null,
+      artifacts: Artifacts.test(),
       fileSystem: fileSystem,
       logger: BufferLogger.test(),
       processManager: FakeProcessManager.any(),
diff --git a/packages/flutter_tools/test/general.shard/dart/generate_synthetic_packages_test.dart b/packages/flutter_tools/test/general.shard/dart/generate_synthetic_packages_test.dart
index 81bbb0e20d6..e086e171321 100644
--- a/packages/flutter_tools/test/general.shard/dart/generate_synthetic_packages_test.dart
+++ b/packages/flutter_tools/test/general.shard/dart/generate_synthetic_packages_test.dart
@@ -2,7 +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 'dart:async';
 
@@ -45,7 +44,7 @@ void main() {
     );
     final Completer<void> completer = Completer<void>();
     final BuildResult exception = BuildResult(success: false, exceptions: <String, ExceptionMeasurement>{
-      'hello': ExceptionMeasurement('hello', const FormatException('illegal character in input string'), null),
+      'hello': ExceptionMeasurement('hello', const FormatException('illegal character in input string'), StackTrace.current),
     });
     final TestBuildSystem buildSystem = TestBuildSystem.all(exception, (Target target, Environment environment) {
       expect(target, const GenerateLocalizationsTarget());
@@ -94,7 +93,7 @@ void main() {
     );
     final Completer<void> completer = Completer<void>();
     final BuildResult exception = BuildResult(success: false, exceptions: <String, ExceptionMeasurement>{
-      'hello': ExceptionMeasurement('hello', const FormatException('illegal character in input string'), null),
+      'hello': ExceptionMeasurement('hello', const FormatException('illegal character in input string'), StackTrace.current),
     });
     final TestBuildSystem buildSystem = TestBuildSystem.all(exception, (Target target, Environment environment) {
       expect(target, const GenerateLocalizationsTarget());
@@ -141,7 +140,7 @@ void main() {
     );
     final Completer<void> completer = Completer<void>();
     final BuildResult exception = BuildResult(success: false, exceptions: <String, ExceptionMeasurement>{
-      'hello': ExceptionMeasurement('hello', const FormatException('illegal character in input string'), null),
+      'hello': ExceptionMeasurement('hello', const FormatException('illegal character in input string'), StackTrace.current),
     });
     final TestBuildSystem buildSystem = TestBuildSystem.all(exception, (Target target, Environment environment) {
       expect(target, const GenerateLocalizationsTarget());