Skip writing unchanged registrant files (#167262)

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This PR adds logic to skip overwriting plugin registrant files if their
contents are unchanged.

Fixes https://github.com/flutter/flutter/issues/134899

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packages/flutter_tools/lib/src/flutter_plugins.dart

@@ -15,6 +15,7 @@ import 'base/file_system.dart';
 import 'base/os.dart';
 import 'base/platform.dart';
 import 'base/template.dart';
+import 'base/utils.dart';
 import 'base/version.dart';
 import 'cache.dart';
 import 'compute_dev_dependencies.dart';
@@ -30,6 +31,14 @@ import 'platform_plugins.dart';
 import 'plugins.dart';
 import 'project.dart';
 
+Future<bool> _fileContentsUnchanged(File file, String renderedTemplate) async {
+  if (!await file.exists()) {
+    return false;
+  }
+  final List<int> fileBytes = await file.readAsBytes();
+  return listEquals(fileBytes, renderedTemplate.codeUnits);
+}
+
 Future<void> _renderTemplateToFile(
   String template,
   Object? context,
@@ -37,6 +46,10 @@ Future<void> _renderTemplateToFile(
   TemplateRenderer templateRenderer,
 ) async {
   final String renderedTemplate = templateRenderer.renderString(template, context);
+  if (await _fileContentsUnchanged(file, renderedTemplate)) {
+    globals.printTrace('Skipping generating ${file.basename} because it is up-to-date.');
+    return;
+  }
   await file.create(recursive: true);
   await file.writeAsString(renderedTemplate);
 }

packages/flutter_tools/test/general.shard/plugins_test.dart

@@ -1455,6 +1455,31 @@ flutter:
         },
       );
 
+      testUsingContext(
+        'Injecting does not overwrite unchanged registrant files',
+        () async {
+          createFakePlugin(fs);
+
+          await injectPlugins(flutterProject, releaseMode: false, linuxPlatform: true);
+
+          final File registrantHeader = linuxProject.managedDirectory.childFile(
+            'generated_plugin_registrant.h',
+          );
+          final DateTime headerLastModified = registrantHeader.lastModifiedSync();
+
+          await injectPlugins(flutterProject, releaseMode: false, linuxPlatform: true);
+
+          // Check that the last modified date is the same.
+          expect(registrantHeader.lastModifiedSync(), headerLastModified);
+        },
+        overrides: <Type, Generator>{
+          FileSystem: () => fs,
+          ProcessManager: () => FakeProcessManager.any(),
+          FeatureFlags: enableExplicitPackageDependencies,
+          Pub: FakePubWithPrimedDeps.new,
+        },
+      );
+
       testUsingContext(
         'Injecting creates generated macos registrant, but does not include Dart-only plugins',
         () async {