anchor.diff:      _tooltip?.remove();
     }
   }
  +
  +class _NullValidator implements NodeValidator {
  +  const _NullValidator();
  +
  +  @override
  +  bool allowsElement(Element element) => true;
  +
  +  @override
  +  bool allowsAttribute(Element element, String attributeName, String value) => true;
  +}
  
positive.code:               if (!dragHappened)
                  tooltip.remove();
              });
          final double left = bar.getBoundingClientRect().left;
          if (left < window.innerWidth / 2.0) {
            tooltip.style.left = '${bar.getBoundingClientRect().right + 5}px';
  
negative.code:         // Used to distinguish between clicks and drags.
          bool dragHappened = false;
          tooltip = new DivElement()
            ..classes.add('metric-value-tooltip')
            ..style.top = '${bar.getBoundingClientRect().top}px'
            ..onMouseDown.listen((_) {
  