post<T> method
Implementation
@override
Future<T> post<T>(Object message) async {
if (closed) {
throw const ClosedException();
}
var completer = Completer<T>.sync();
var id = _nextId++;
handlers[id] = completer;
sendPort.send(_RequestMessage(id, message, receivePort.sendPort));
return await completer.future;
}