rebuild the asset bundle if a file has been modified between flutter test runs (#143569)

Fixes https://github.com/flutter/flutter/issues/143513
Should be cherry-picked to beta.

packages/flutter_tools/lib/src/commands/test.dart

@@ -620,7 +620,10 @@ class TestCommand extends FlutterCommand with DeviceBasedDevelopmentArtifacts {
       return true;
     }
 
-    for (final DevFSFileContent entry in entries.values.whereType<DevFSFileContent>()) {
+    final Iterable<DevFSFileContent> files = entries.values
+      .map((AssetBundleEntry asset) => asset.content)
+      .whereType<DevFSFileContent>();
+    for (final DevFSFileContent entry in files) {
       // Calling isModified to access file stats first in order for isModifiedAfter
       // to work.
       if (entry.isModified && entry.isModifiedAfter(lastModified)) {

packages/flutter_tools/test/commands.shard/hermetic/test_test.dart

@@ -955,6 +955,41 @@ dev_dependencies:
     DeviceManager: () => _FakeDeviceManager(<Device>[]),
   });
 
+  testUsingContext('Rebuild the asset bundle if an asset file has changed since previous build', () async {
+    final FakeFlutterTestRunner testRunner = FakeFlutterTestRunner(0);
+    fs.file('asset.txt').writeAsStringSync('1');
+    fs.file('pubspec.yaml').writeAsStringSync('''
+flutter:
+  assets:
+    - asset.txt
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+  integration_test:
+    sdk: flutter''');
+    final TestCommand testCommand = TestCommand(testRunner: testRunner);
+    final CommandRunner<void> commandRunner = createTestCommandRunner(testCommand);
+
+    await commandRunner.run(const <String>[
+      'test',
+      '--no-pub',
+    ]);
+
+    fs.file('asset.txt').writeAsStringSync('2');
+
+    await commandRunner.run(const <String>[
+      'test',
+      '--no-pub',
+    ]);
+
+    final String fileContent = fs.file(globals.fs.path.join('build', 'unit_test_assets', 'asset.txt')).readAsStringSync();
+    expect(fileContent, '2');
+  }, overrides: <Type, Generator>{
+    FileSystem: () => fs,
+    ProcessManager: () => FakeProcessManager.empty(),
+    DeviceManager: () => _FakeDeviceManager(<Device>[]),
+  });
+
   group('Fatal Logs', () {
     testUsingContext("doesn't fail when --fatal-warnings is set and no warning output", () async {
       final FakeFlutterTestRunner testRunner = FakeFlutterTestRunner(0);