anchor.diff:    Listener(querySelector('#listener-2')!);
   
     // Init Event fields.
  -  LabelElement? fireLabelA = querySelector('#fire-label-a') as LabelElement?;
  -  LabelElement? fireLabelB = querySelector('#fire-label-b') as LabelElement?;
  +  LabelElement fireLabelA = querySelector('#fire-label-a') as LabelElement;
  +  LabelElement fireLabelB = querySelector('#fire-label-b') as LabelElement;
     ButtonElement fireButtonA = querySelector("#fire-button-a") as ButtonElement;
     ButtonElement fireButtonB = querySelector("#fire-button-b") as ButtonElement;
   
  
positive.code:     // Fire Event B
      // -------------------------------------------------
      eventBus.fire(MyEventB('Received Event B [$counterB]'));
      fireLabelB!.text = '--> fired [$counterB]';
      counterB++;
    });
  }
  
negative.code:     // Fire Event A
      // -------------------------------------------------
      eventBus.fire(MyEventA('Received Event A [$counterA]'));
      fireLabelA.text = '--> fired [$counterA]';
      counterA++;
    });
    fireButtonB.onClick.listen((_) {
  