void postMessage(message, [ List<MessagePort> transfer ])

Source 

void postMessage(/*SerializedScriptValue*/ message,
    [List<MessagePort> transfer]) {
  if (transfer != null) {
    _blink.BlinkClient.instance.postMessage_Callback_2_(
        this, convertDartToNative_SerializedScriptValue(message), transfer);
    return;
  }
  _blink.BlinkClient.instance.postMessage_Callback_1_(
      this, convertDartToNative_SerializedScriptValue(message));
  return;
}