make hot reload reflect changes to asset transformer configurations (#144660)

In service of https://github.com/flutter/flutter/issues/143348

This PR makes hot reloads reflect changes to transformer configurations under the `assets` section in pubspec.yaml.
This PR is optimized for ease of implementation, and should not be merged as-is. If it is merged as-is, seriously consider creating a tech debt issue and assigning it to someone to make sure it gets resolved.

packages/flutter_tools/lib/src/asset.dart

@@ -14,6 +14,7 @@ import 'base/deferred_component.dart';
 import 'base/file_system.dart';
 import 'base/logger.dart';
 import 'base/platform.dart';
+import 'base/utils.dart';
 import 'build_info.dart';
 import 'cache.dart';
 import 'convert.dart';
@@ -84,9 +85,8 @@ enum AssetKind {
 
 /// Contains all information about an asset needed by tool the to prepare and
 /// copy an asset file to the build output.
-@immutable
 final class AssetBundleEntry {
-  const AssetBundleEntry(this.content, {
+  AssetBundleEntry(this.content, {
     required this.kind,
     required this.transformers,
   });
@@ -96,6 +96,10 @@ final class AssetBundleEntry {
   final List<AssetTransformerEntry> transformers;
 
   Future<List<int>> contentsAsBytes() => content.contentsAsBytes();
+
+  bool hasEquivalentConfigurationWith(AssetBundleEntry other) {
+    return listEquals(transformers, other.transformers);
+  }
 }
 
 abstract class AssetBundle {
@@ -425,11 +429,11 @@ class ManifestAssetBundle implements AssetBundle {
         final File variantFile = variant.lookupAssetFile(_fileSystem);
         inputFiles.add(variantFile);
         assert(variantFile.existsSync());
-        entries[variant.entryUri.path] ??= AssetBundleEntry(
+        _setIfConfigurationChanged(entries, variant.entryUri.path, AssetBundleEntry(
           DevFSFileContent(variantFile),
           kind: variant.kind,
           transformers: variant.transformers,
-        );
+        ));
       }
     }
     // Save the contents of each deferred component image, image variant, and font
@@ -459,11 +463,11 @@ class ManifestAssetBundle implements AssetBundle {
         for (final _Asset variant in assetsMap[asset]!) {
           final File variantFile = variant.lookupAssetFile(_fileSystem);
           assert(variantFile.existsSync());
-          deferredComponentsEntries[componentName]![variant.entryUri.path] ??= AssetBundleEntry(
+          _setIfConfigurationChanged(deferredComponentsEntries[componentName]!, variant.entryUri.path, AssetBundleEntry(
             DevFSFileContent(variantFile),
             kind: AssetKind.regular,
             transformers: variant.transformers,
-          );
+          ));
         }
       }
     }
@@ -563,6 +567,13 @@ class ManifestAssetBundle implements AssetBundle {
     return true;
   }
 
+  void _setIfConfigurationChanged(Map<String, AssetBundleEntry> entryMap, String key, AssetBundleEntry entry,) {
+    final AssetBundleEntry? existingEntry = entryMap[key];
+    if (existingEntry == null || !entry.hasEquivalentConfigurationWith(existingEntry)) {
+      entryMap[key] = entry;
+    }
+  }
+
   void _setLicenseIfChanged(
     String combinedLicenses,
     TargetPlatform? targetPlatform,

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

@@ -428,6 +428,73 @@ flutter:
         ),
       );
     });
+
+    testWithoutContext("AssetBundleEntry::content::isModified is true when an asset's transformers change in between builds", () async {
+      final FileSystem fileSystem = MemoryFileSystem.test();
+
+      fileSystem.file('my-asset.txt').createSync();
+
+      final BufferLogger logger = BufferLogger.test();
+      final FakePlatform platform = FakePlatform();
+      fileSystem.file('.packages').createSync();
+      fileSystem.file('pubspec.yaml')
+        ..createSync()
+        ..writeAsStringSync(r'''
+name: example
+flutter:
+  assets:
+    - path: my-asset.txt
+      transformers:
+        - package: my-transformer-one
+''');
+      final ManifestAssetBundle bundle = ManifestAssetBundle(
+        logger: logger,
+        fileSystem: fileSystem,
+        platform: platform,
+        flutterRoot: Cache.defaultFlutterRoot(
+          platform: platform,
+          fileSystem: fileSystem,
+          userMessages: UserMessages(),
+        ),
+      );
+
+      await bundle.build(
+        packagesPath: '.packages',
+        flutterProject: FlutterProject.fromDirectoryTest(
+          fileSystem.currentDirectory,
+        ),
+      );
+
+      expect(bundle.entries['my-asset.txt']!.content.isModified, isTrue);
+
+      await bundle.build(
+        packagesPath: '.packages',
+        flutterProject: FlutterProject.fromDirectoryTest(
+          fileSystem.currentDirectory,
+        ),
+      );
+
+      expect(bundle.entries['my-asset.txt']!.content.isModified, isFalse);
+
+      fileSystem.file('pubspec.yaml').writeAsStringSync(r'''
+name: example
+flutter:
+  assets:
+    - path: my-asset.txt
+      transformers:
+        - package: my-transformer-one
+        - package: my-transformer-two
+''');
+
+      await bundle.build(
+        packagesPath: '.packages',
+        flutterProject: FlutterProject.fromDirectoryTest(
+          fileSystem.currentDirectory,
+        ),
+      );
+
+      expect(bundle.entries['my-asset.txt']!.content.isModified, isTrue);
+    });
   });
 
   group('AssetBundle.build (web builds)', () {