anchor.diff:    Stream<ServiceActionEvent> onIsolateResponse(String callId) {
       // TODO(ericarnold): Implement
       StreamSubscription subscription;
  -    StreamController<ActionEvent> controller =
  -        new StreamController<ActionEvent>();
  -    subscription = onIsolateMessage.listen((ActionEvent event){
  +    StreamController<ServiceActionEvent> controller =
  +        new StreamController<ServiceActionEvent>();
  +    subscription = onIsolateMessage.listen((ServiceActionEvent event){
         if (event.response && event.callId == callId) {
           subscription.cancel();
           controller..add(event)..close();
  
positive.code: 
  class ExampleService extends Service {
    String serviceId = "example";
    ExampleService(Services services, _IsolateHandler handler)
        : super(services, handler);
  }
  
negative.code:     return _isolateHandler.onceIsolateReady.then((_){
        _isolateHandler.sendAction(serviceId, actionId, callId);
        return _isolateHandler.onIsolateResponse(callId).first;
      }).then((ServiceActionEvent event){
        print("pong");
        return new Future.value("poong");
      });
  