void sendPlatformMessage(String name, ByteData data, PlatformMessageResponseCallback callback)

Sends a message to a platform-specific plugin.

The name parameter determines which plugin receives the message. The data parameter contains the message payload and is typically UTF-8 encoded JSON but can be arbitrary data. If the plugin replies to the message, callback will be called with the response.

Source 

void sendPlatformMessage(String name,
                         ByteData data,
                         PlatformMessageResponseCallback callback) {
  _sendPlatformMessage(name, callback, data);
}