anchor.diff:  /// register handlers for incoming messages with [configure].
   class FirebaseMessaging {
     factory FirebaseMessaging() => _instance;
  +
  +  @visibleForTesting
  +  FirebaseMessaging.private(MethodChannel channel, Platform platform)
  +      : _channel = channel,
  +        _platform = platform;
  +
  +  static final FirebaseMessaging _instance = FirebaseMessaging.private(
  +      const MethodChannel('plugins.flutter.io/firebase_messaging'),
  +      const LocalPlatform());
   
     /// Setup method channel to handle Firebase Cloud Messages received while
     /// the Flutter app is not active. The handle for this method is generated
  
positive.code:     // know that it can start scheduling handling messages.
      backgroundChannel.invokeMethod<void>('FcmDartService#initialized');
    }
  
    @visibleForTesting
    FirebaseMessaging.private(MethodChannel channel, Platform platform)
        : _channel = channel,
          _platform = platform;
  
    static final FirebaseMessaging _instance = FirebaseMessaging.private(
        const MethodChannel('plugins.flutter.io/firebase_messaging'),
        const LocalPlatform());
  
    final MethodChannel _channel;
    final Platform _platform;
  
negative.code:     backgroundChannel.setMethodCallHandler((MethodCall call) async {
        if (call.method == 'handleBackgroundMessage') {
          final CallbackHandle handle =
          CallbackHandle.fromRawHandle(call.arguments['handle']);
          final Function handlerFunction =
          PluginUtilities.getCallbackFromHandle(handle);
          try {
            await handlerFunction(
                Map<String, dynamic>.from(call.arguments['message']));
  