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

Source 

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