[flutter_tools] ensure build fails if asset files are missing (#54233)

Ensure build fails if asset files are missing and that stderr messages are forwarded through Gradle

dev/devicelab/bin/tasks/gradle_plugin_light_apk_test.dart

@@ -190,6 +190,19 @@ Future<void> main() async {
               result);
       });
 
+      await runProjectTest((FlutterProject project) async {
+        section('gradlew assembleDebug forwards stderr');
+        await project.introducePubspecError();
+                final ProcessResult result =
+            await project.resultOfGradleTask('assembleRelease');
+        if (result.exitCode == 0)
+          throw failure(
+              'Gradle did not exit with error as expected', result);
+        final String output = '${result.stdout}\n${result.stderr}';
+        if (!output.contains('No file or variants found for asset: lib/gallery/example_code.dart.'))
+          throw failure(output, result);
+      });
+
       await runProjectTest((FlutterProject project) async {
         section('flutter build apk on build script with error');
         await project.introduceError();

dev/devicelab/lib/framework/apk_utils.dart

@@ -299,6 +299,20 @@ android {
     await buildScript.writeAsString((await buildScript.readAsString()).replaceAll('buildTypes', 'builTypes'));
   }
 
+  Future<void> introducePubspecError() async {
+    final File pubspec = File(
+      path.join(parent.path, 'hello', 'pubspec.yaml')
+    );
+    final String contents = pubspec.readAsStringSync();
+    final String newContents = contents.replaceFirst('# The following section is specific to Flutter.\nflutter:\n', '''
+flutter:
+  assets:
+    - lib/gallery/example_code.dart
+
+''');
+    pubspec.writeAsStringSync(newContents);
+  }
+
   Future<void> runGradleTask(String task, {List<String> options}) async {
     return _runGradleTask(workingDirectory: androidPath, task: task, options: options);
   }

packages/flutter_tools/gradle/flutter.gradle

@@ -881,6 +881,7 @@ abstract class BaseFlutterTask extends DefaultTask {
             ruleNames = targetPlatformValues.collect { "android_aot_bundle_${buildMode}_$it" }
         }
         project.exec {
+            logging.captureStandardError LogLevel.ERROR
             executable flutterExecutable.absolutePath
             workingDir sourceDir
             if (localEngine != null) {

packages/flutter_tools/lib/src/build_system/targets/assets.dart

@@ -16,14 +16,19 @@ import 'icon_tree_shaker.dart';
 /// A helper function to copy an asset bundle into an [environment]'s output
 /// directory.
 ///
+/// Throws [Exception] if [AssetBundle.build] returns a non-zero exit code.
+///
 /// Returns a [Depfile] containing all assets used in the build.
 Future<Depfile> copyAssets(Environment environment, Directory outputDirectory) async {
   final File pubspecFile =  environment.projectDir.childFile('pubspec.yaml');
   final AssetBundle assetBundle = AssetBundleFactory.instance.createBundle();
-  await assetBundle.build(
+  final int resultCode = await assetBundle.build(
     manifestPath: pubspecFile.path,
     packagesPath: environment.projectDir.childFile('.packages').path,
   );
+  if (resultCode != 0) {
+    throw Exception('Failed to bundle asset files.');
+  }
   final Pool pool = Pool(kMaxOpenFiles);
   final List<File> inputs = <File>[
     // An asset manifest with no assets would have zero inputs if not

packages/flutter_tools/test/general.shard/build_system/targets/assets_test.dart

@@ -97,6 +97,26 @@ flutter:
     ProcessManager: () => FakeProcessManager.any(),
     Platform: () => platform,
   });
+
+  testUsingContext('Throws exception if pubspec contains missing files', () async {
+    fileSystem.file('pubspec.yaml')
+      ..createSync()
+      ..writeAsStringSync('''
+name: example
+
+flutter:
+  assets:
+    - assets/foo/bar2.png
+
+''');
+
+    expect(() async => await const CopyAssets().build(environment),
+      throwsA(isA<Exception>()));
+  }, overrides: <Type, Generator>{
+    FileSystem: () => fileSystem,
+    ProcessManager: () => FakeProcessManager.any(),
+    Platform: () => platform,
+  });
 }
 
 class MockArtifacts extends Mock implements Artifacts {}