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