anchor.diff:      backgroundChannel.setMethodCallHandler((MethodCall call) async {
         if (call.method == 'handleBackgroundMessage') {
           final CallbackHandle handle =
  -            CallbackHandle.fromRawHandle(call.arguments['handle']);
  +        CallbackHandle.fromRawHandle(call.arguments['handle']);
           final Function handlerFunction =
  -            PluginUtilities.getCallbackFromHandle(handle);
  +        PluginUtilities.getCallbackFromHandle(handle);
           try {
             await handlerFunction(
                 Map<String, dynamic>.from(call.arguments['message']));
  
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: /// 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
  