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