void setMockStringMessageHandler(String channel, Future<String> handler(String message))

Sets a message handler that intercepts outgoing messages in string form.

The given callback will replace the currently registered callback (if any). To remove the mock handler, pass null as the handler argument.

Source 

static void setMockStringMessageHandler(String channel, Future<String> handler(String message)) {
  setMockBinaryMessageHandler(channel, (ByteData message) async {
    return _encodeUTF8(await handler(_decodeUTF8(message)));
  });
}