anchor.diff:      assert(buttonCont.getDistributedNodes().isNotEmpty);
       _button = buttonCont.getDistributedNodes().first;
       _button
  +        ..onMouseDown.listen(_mouseDownHandler)
           ..onClick.listen(clickHandler)
           ..onFocus.listen(focusHandler)
           ..onBlur.listen(blurHandler);
  
positive.code:     }
    }
  
    void clickHandler(Event e) {
      print('clickHandler');
      _toggle(!opened);
    }
  
    void focusHandler(Event e) {
      print('focusHandler');
      _toggle(true);
      scheduleMicrotask(() {
        print('scheduleMicrotask');
      });
    }
  
    void blurHandler(FocusEvent e) {
      var target = e.relatedTarget;
  
negative.code:   SparkButton _button;
    SparkOverlay _overlay;
    SparkMenu _menu;
    bool _mouseDownOpened;
  
    SparkMenuButton.created(): super.created();
  
  