add default-flavor field to flutter pubspec, which will be used as the flavor in flutter build/run if --flavor is not provided (#147968)

This PR adds a new flag `default-flavor` in the `flutter` section of `pubspec.yaml`. It allows developers of multi-flavor android apps to specify a default flavor to be used for `flutter run`, `flutter build` etc.
Using `flutter run` on flavored apps already works without specifying `--flavor` already works on iOS (it defaults to the `runner` schema), so I (and others in #22856) figured this would be nice to have.

fixes #22856

packages/flutter_tools/lib/src/flutter_manifest.dart

@@ -388,6 +388,8 @@ class FlutterManifest {
     }
     return value;
   }
+
+  String? get defaultFlavor => _flutterDescriptor['default-flavor'] as String?;
 }
 
 class Font {
@@ -557,6 +559,10 @@ void _validateFlutter(YamlMap? yaml, List<String> errors) {
         if (yamlValue is! bool) {
           errors.add('Expected "$yamlKey" to be a bool, but got $yamlValue (${yamlValue.runtimeType}).');
         }
+      case 'default-flavor':
+        if (yamlValue is! String) {
+          errors.add('Expected "$yamlKey" to be a string, but got $yamlValue (${yamlValue.runtimeType}).');
+        }
       default:
         errors.add('Unexpected child "$yamlKey" found under "flutter".');
         break;

packages/flutter_tools/lib/src/runner/flutter_command.dart

@@ -1094,7 +1094,8 @@ abstract class FlutterCommand extends Command<void> {
       'flavor',
       help: 'Build a custom app flavor as defined by platform-specific build setup.\n'
             'Supports the use of product flavors in Android Gradle scripts, and '
-            'the use of custom Xcode schemes.',
+            'the use of custom Xcode schemes.\n'
+            'Overrides the value of the "default-flavor" entry in the flutter pubspec.',
     );
   }
 
@@ -1283,7 +1284,9 @@ abstract class FlutterCommand extends Command<void> {
       }
     }
 
-    final String? flavor = argParser.options.containsKey('flavor') ? stringArg('flavor') : null;
+    final String? defaultFlavor = FlutterProject.current().manifest.defaultFlavor;
+    final String? cliFlavor = argParser.options.containsKey('flavor') ? stringArg('flavor') : null;
+    final String? flavor = cliFlavor ?? defaultFlavor;
     if (flavor != null) {
       if (globals.platform.environment['FLUTTER_APP_FLAVOR'] != null) {
         throwToolExit('FLUTTER_APP_FLAVOR is used by the framework and cannot be set in the environment.');

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

@@ -36,6 +36,7 @@ void main() {
     expect(flutterManifest.fonts, isEmpty);
     expect(flutterManifest.assets, isEmpty);
     expect(flutterManifest.additionalLicenses, isEmpty);
+    expect(flutterManifest.defaultFlavor, null);
   });
 
   testWithoutContext('FlutterManifest is null when the pubspec.yaml file is not a map', () async {
@@ -1448,6 +1449,37 @@ flutter:
 
     expect(flutterManifest.disabledSwiftPackageManager, false);
   });
+
+  testWithoutContext('FlutterManifest can parse default flavor', () async {
+    const String manifest = '''
+name: test
+flutter:
+    default-flavor: prod
+''';
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
+      manifest,
+      logger: BufferLogger.test(),
+    );
+
+    expect(flutterManifest, isNotNull);
+    expect(flutterManifest!.defaultFlavor, 'prod');
+  });
+
+  testWithoutContext('FlutterManifest fails on invalid default flavor', () async {
+    const String manifest = '''
+name: test
+flutter:
+    default-flavor: 3
+''';
+
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
+      manifest,
+      logger: logger,
+    );
+
+    expect(flutterManifest, null);
+    expect(logger.errorText, 'Expected "default-flavor" to be a string, but got 3 (int).\n');
+  });
 }
 
 Matcher matchesManifest({

packages/flutter_tools/test/general.shard/runner/flutter_command_test.dart

@@ -1217,6 +1217,45 @@ void main() {
         FileSystem: () => fileSystem,
         ProcessManager: () => FakeProcessManager.any(),
       });
+
+      testUsingContext('CLI option overrides default flavor from manifest', () async {
+        final File pubspec = fileSystem.file('pubspec.yaml');
+        await pubspec.create();
+        await pubspec.writeAsString('''
+name: test
+flutter:
+  default-flavor: foo
+        ''');
+
+        final DummyFlutterCommand flutterCommand = DummyFlutterCommand();
+        final BuildInfo buildInfo = await flutterCommand.getBuildInfo(forcedBuildMode: BuildMode.debug);
+        expect(buildInfo.flavor, 'foo');
+      }, overrides: <Type, Generator>{
+        FileSystem: () => fileSystem,
+        ProcessManager: () => FakeProcessManager.empty(),
+      });
+
+      testUsingContext('tool loads default flavor from manifest, but cli overrides', () async {
+        final File pubspec = fileSystem.file('pubspec.yaml');
+        await pubspec.create();
+        await pubspec.writeAsString('''
+name: test
+flutter:
+  default-flavor: foo
+        ''');
+
+        final DummyFlutterCommand flutterCommand = DummyFlutterCommand(commandFunction: () async {
+          return FlutterCommandResult.success();
+        },);
+        flutterCommand.usesFlavorOption();
+        final CommandRunner<void> runner =  createTestCommandRunner(flutterCommand);
+        await runner.run(<String>['dummy', '--flavor', 'bar']);
+        final BuildInfo buildInfo = await flutterCommand.getBuildInfo(forcedBuildMode: BuildMode.debug);
+        expect(buildInfo.flavor, 'bar');
+      }, overrides: <Type, Generator>{
+        FileSystem: () => fileSystem,
+        ProcessManager: () => FakeProcessManager.empty(),
+      });
     });
   });
 }