anchor.diff:      assert(buttonCont.getDistributedNodes().isNotEmpty);
       _button = buttonCont.getDistributedNodes().first;
       _button
  -        ..onMouseDown.listen(_mouseDownHandler)
  +        ..onMouseDown.listen(mouseDownHandler)
           ..onClick.listen(clickHandler)
           ..onFocus.listen(focusHandler)
           ..onBlur.listen(blurHandler);
  
positive.code:     }
    }
  
    void _mouseDownHandler(Event e) {
      _mouseDownOpened = opened;
    }
  
    void clickHandler(Event e) => _toggle(!_mouseDownOpened);
  
    void focusHandler(Event e) => _toggle(true);
  
negative.code:   SparkButton _button;
    SparkOverlay _overlay;
    SparkMenu _menu;
  
    /**
     * The state of the menu on `mouseDown`.
     */
    bool _mouseDownOpened;
  
    SparkMenuButton.created(): super.created();
  