Add API to read flavor from framework at run time (#134179)

Resolves #128046.

Adds a services API that allows flutter app developers to write app code that determines `--flavor` the app was built with.

This is implemented by having the tool adding the value of `--flavor` to its list of dart environment declarations, which will be available to the app at run time. Specifically,`FLUTTER_APP_FLAVOR` is set. I chose this implementation for its simplicity. There is some precedent for this, but only for web ([example](https://github.com/flutter/flutter/blob/cd2f3f5e78409027d3c9014172708e4dec7f2185/packages/flutter_tools/lib/src/runner/flutter_command.dart#L1231)).

dev/integration_tests/flavors/integration_test/integration_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flavors/main.dart' as app;
+import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:integration_test/integration_test.dart';
 
@@ -16,6 +17,7 @@ void main() {
       await tester.pumpAndSettle();
 
       expect(find.text('paid'), findsOneWidget);
+      expect(appFlavor, 'paid');
     });
   });
 }

packages/flutter/lib/services.dart

@@ -19,6 +19,7 @@ export 'src/services/browser_context_menu.dart';
 export 'src/services/clipboard.dart';
 export 'src/services/debug.dart';
 export 'src/services/deferred_component.dart';
+export 'src/services/flavor.dart';
 export 'src/services/font_loader.dart';
 export 'src/services/haptic_feedback.dart';
 export 'src/services/hardware_keyboard.dart';

packages/flutter/lib/src/services/flavor.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/// The flavor this app was built with.
+///
+/// This is equivalent to the value argued to the `--flavor` option at build time.
+/// This will be `null` if the `--flavor` option was not provided.
+const String? appFlavor = String.fromEnvironment('FLUTTER_APP_FLAVOR') != '' ?
+  String.fromEnvironment('FLUTTER_APP_FLAVOR') : null;

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

@@ -1247,10 +1247,20 @@ abstract class FlutterCommand extends Command<void> {
       }
     }
 
+    final String? flavor = argParser.options.containsKey('flavor') ? stringArg('flavor') : null;
+    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.');
+      }
+      if (dartDefines.any((String define) => define.startsWith('FLUTTER_APP_FLAVOR'))) {
+        throwToolExit('FLUTTER_APP_FLAVOR is used by the framework and cannot be '
+          'set using --${FlutterOptions.kDartDefinesOption} or --${FlutterOptions.kDartDefineFromFileOption}');
+      }
+      dartDefines.add('FLUTTER_APP_FLAVOR=$flavor');
+    }
+
     return BuildInfo(buildMode,
-      argParser.options.containsKey('flavor')
-        ? stringArg('flavor')
-        : null,
+      flavor,
       trackWidgetCreation: trackWidgetCreation,
       frontendServerStarterPath: argParser.options
               .containsKey(FlutterOptions.kFrontendServerStarterPath)

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

@@ -11,11 +11,14 @@ import 'package:flutter_tools/src/base/common.dart';
 import 'package:flutter_tools/src/base/error_handling_io.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/io.dart';
+import 'package:flutter_tools/src/base/logger.dart';
+import 'package:flutter_tools/src/base/platform.dart';
 import 'package:flutter_tools/src/base/signals.dart';
 import 'package:flutter_tools/src/base/time.dart';
 import 'package:flutter_tools/src/base/user_messages.dart';
 import 'package:flutter_tools/src/build_info.dart';
 import 'package:flutter_tools/src/cache.dart';
+import 'package:flutter_tools/src/commands/run.dart';
 import 'package:flutter_tools/src/dart/pub.dart';
 import 'package:flutter_tools/src/device.dart';
 import 'package:flutter_tools/src/globals.dart' as globals;
@@ -700,6 +703,67 @@ void main() {
         expect(testLogger.statusText, contains(UserMessages().flutterSpecifyDevice));
       });
     });
+
+    group('--flavor', () {
+      late _TestDeviceManager testDeviceManager;
+      late Logger logger;
+      late FileSystem fileSystem;
+
+      setUp(() {
+        logger = BufferLogger.test();
+        testDeviceManager = _TestDeviceManager(logger: logger);
+        fileSystem = MemoryFileSystem.test();
+      });
+
+      testUsingContext("tool exits when FLUTTER_APP_FLAVOR is already set in user's environment", () async {
+        fileSystem.file('lib/main.dart').createSync(recursive: true);
+        fileSystem.file('pubspec.yaml').createSync();
+        fileSystem.file('.packages').createSync();
+
+        final FakeDevice device = FakeDevice('name', 'id');
+        testDeviceManager.devices = <Device>[device];
+        final _TestRunCommandThatOnlyValidates command = _TestRunCommandThatOnlyValidates();
+        final CommandRunner<void> runner =  createTestCommandRunner(command);
+
+        expect(runner.run(<String>['run', '--no-pub', '--no-hot', '--flavor=strawberry']),
+          throwsToolExit(message: 'FLUTTER_APP_FLAVOR is used by the framework and cannot be set in the environment.'));
+
+      }, overrides: <Type, Generator>{
+        DeviceManager: () => testDeviceManager,
+        Platform: () => FakePlatform(
+          environment: <String, String>{
+            'FLUTTER_APP_FLAVOR': 'I was already set'
+          }
+        ),
+        Cache: () => Cache.test(processManager: FakeProcessManager.any()),
+        FileSystem: () => fileSystem,
+        ProcessManager: () => FakeProcessManager.any(),
+      });
+
+      testUsingContext('tool exits when FLUTTER_APP_FLAVOR is set in --dart-define or --dart-define-from-file', () async {
+        fileSystem.file('lib/main.dart').createSync(recursive: true);
+        fileSystem.file('pubspec.yaml').createSync();
+        fileSystem.file('.packages').createSync();
+        fileSystem.file('config.json')..createSync()..writeAsStringSync('{"FLUTTER_APP_FLAVOR": "strawberry"}');
+
+        final FakeDevice device = FakeDevice('name', 'id');
+        testDeviceManager.devices = <Device>[device];
+        final _TestRunCommandThatOnlyValidates command = _TestRunCommandThatOnlyValidates();
+        final CommandRunner<void> runner =  createTestCommandRunner(command);
+
+        expect(runner.run(<String>['run', '--dart-define=FLUTTER_APP_FLAVOR=strawberry', '--no-pub', '--no-hot', '--flavor=strawberry']),
+          throwsToolExit(message: 'FLUTTER_APP_FLAVOR is used by the framework and cannot be set using --dart-define or --dart-define-from-file'));
+
+        expect(runner.run(<String>['run', '--dart-define-from-file=config.json', '--no-pub', '--no-hot', '--flavor=strawberry']),
+          throwsToolExit(message: 'FLUTTER_APP_FLAVOR is used by the framework and cannot be set using --dart-define or --dart-define-from-file'));
+      }, overrides: <Type, Generator>{
+        DeviceManager: () => testDeviceManager,
+        Platform: () => FakePlatform(),
+        Cache: () => Cache.test(processManager: FakeProcessManager.any()),
+        FileSystem: () => fileSystem,
+        ProcessManager: () => FakeProcessManager.any(),
+      });
+    });
   });
 }
 
@@ -853,3 +917,22 @@ class FakePub extends Fake implements Pub {
     PubOutputMode outputMode = PubOutputMode.all,
   }) async { }
 }
+
+class _TestDeviceManager extends DeviceManager {
+  _TestDeviceManager({required super.logger});
+  List<Device> devices = <Device>[];
+
+  @override
+  List<DeviceDiscovery> get deviceDiscoverers {
+    final FakePollingDeviceDiscovery discoverer = FakePollingDeviceDiscovery();
+    devices.forEach(discoverer.addDevice);
+    return <DeviceDiscovery>[discoverer];
+  }
+}
+
+class _TestRunCommandThatOnlyValidates extends RunCommand {
+  @override
+  Future<FlutterCommandResult> runCommand() async {
+    return FlutterCommandResult.success();
+  }
+}