[flutter_tools] don't recreate license, manifest, asset if unchanged (#56107)

Avoid syncing unchanged versions of these files during hot restart/reload. This happens if the asset bundle needs to be built but produces identical manifests/licenses due to only asset contents changing

packages/flutter_tools/lib/src/asset.dart

@@ -260,20 +260,31 @@ class ManifestAssetBundle implements AssetBundle {
       _wildcardDirectories[uri] ??= globals.fs.directory(uri);
     }
 
-    entries[_assetManifestJson] = _createAssetManifest(assetVariants);
-
-    entries[kFontManifestJson] = DevFSStringContent(json.encode(fonts));
-
-    // TODO(ianh): Only do the following line if we've changed packages or if our LICENSE file changed
+    final DevFSStringContent assetManifest  = _createAssetManifest(assetVariants);
+    final DevFSStringContent fontManifest = DevFSStringContent(json.encode(fonts));
     final LicenseResult licenseResult = licenseCollector.obtainLicenses(packageConfig);
-    entries[_license] = DevFSStringContent(licenseResult.combinedLicenses);
+    final DevFSStringContent licenses = DevFSStringContent(licenseResult.combinedLicenses);
     additionalDependencies = licenseResult.dependencies;
 
+    _setIfChanged(_assetManifestJson, assetManifest);
+    _setIfChanged(kFontManifestJson, fontManifest);
+    _setIfChanged(_license, licenses);
     return 0;
   }
 
   @override
   List<File> additionalDependencies = <File>[];
+
+  void _setIfChanged(String key, DevFSStringContent content) {
+    if (!entries.containsKey(key)) {
+      entries[key] = content;
+      return;
+    }
+    final DevFSStringContent oldContent = entries[key] as DevFSStringContent;
+    if (oldContent.string != content.string) {
+      entries[key] = content;
+    }
+  }
 }
 
 @immutable
@@ -469,7 +480,7 @@ int _byBasename(_Asset a, _Asset b) {
   return a.assetFile.basename.compareTo(b.assetFile.basename);
 }
 
-DevFSContent _createAssetManifest(Map<_Asset, List<_Asset>> assetVariants) {
+DevFSStringContent _createAssetManifest(Map<_Asset, List<_Asset>> assetVariants) {
   final Map<String, List<String>> jsonObject = <String, List<String>>{};
 
   // necessary for making unit tests deterministic

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

@@ -192,6 +192,37 @@ flutter:
     verify(mockDirectory.createSync(recursive: true)).called(1);
     expect(testLogger.errorText, contains('ABCD'));
   });
+
+  testUsingContext('does not unnecessarily recreate asset manifest, font manifest, license', () async {
+    globals.fs.file('.packages').createSync();
+    globals.fs.file(globals.fs.path.join('assets', 'foo', 'bar.txt')).createSync(recursive: true);
+    globals.fs.file('pubspec.yaml')
+      ..createSync()
+      ..writeAsStringSync(r'''
+name: example
+flutter:
+assets:
+  - assets/foo/bar.txt
+''');
+    final AssetBundle bundle = AssetBundleFactory.instance.createBundle();
+    await bundle.build(manifestPath: 'pubspec.yaml');
+
+    final DevFSStringContent assetManifest = bundle.entries['AssetManifest.json']
+      as DevFSStringContent;
+    final DevFSStringContent fontManifest = bundle.entries['FontManifest.json']
+      as DevFSStringContent;
+    final DevFSStringContent license = bundle.entries['LICENSE']
+      as DevFSStringContent;
+
+    await bundle.build(manifestPath: 'pubspec.yaml');
+
+    expect(assetManifest, bundle.entries['AssetManifest.json']);
+    expect(fontManifest, bundle.entries['FontManifest.json']);
+    expect(license, bundle.entries['LICENSE']);
+  }, overrides: <Type, Generator>{
+    FileSystem: () => MemoryFileSystem.test(),
+    ProcessManager: () => FakeProcessManager.any(),
+  });
 }
 
 class MockDirectory extends Mock implements Directory {}