anchor.diff:    ///
     /// This is intended for testing. Messages intercepted in this manner are not
     /// sent to platform plugins.
  -  void setMockMessageHandler(Future<T> handler(T message)) {
  +  void setMockMessageHandler(Future<T> Function(T message) handler) {
       if (handler == null) {
         binaryMessenger.setMockMessageHandler(name, null);
       } else {
  
positive.code:   ///
    /// This method is useful for tests or test harnesses that want to assert the
    /// handler for the specified channel has not been altered by a previous test.
    bool checkMockMethodCallHandler(Future<dynamic> handler(MethodCall call)) => _methodChannelMockHandlers[this] == handler;
  
    Future<ByteData?> _handleAsMethodCall(ByteData? message, Future<dynamic> handler(MethodCall call)) async {
      final MethodCall call = codec.decodeMethodCall(message);
  
negative.code:   /// return value of the call. The value will be encoded using
    /// [MethodCodec.encodeSuccessEnvelope], to act as if platform plugin had
    /// returned that value.
    void setMockMethodCallHandler(Future<dynamic> Function(MethodCall call)? handler) {
      _methodChannelMockHandlers[this] = handler;
      binaryMessenger.setMockMessageHandler(
        name,
  