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

Source 

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