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:         // Used to distinguish between clicks and drags.
          bool dragHappened = false;
          tooltip = new DivElement()
            ..text = '${value.value}$unit\n'
              'Flutter revision: ${value.revision}\n'
              'Recorded on: ${new DateTime.fromMillisecondsSinceEpoch(value.createTimestamp)}\n'
              'Goal: $goal$unit\n'
              'Baseline: $baseline$unit'
            ..classes.add('metric-value-tooltip')
            ..style.top = '${bar.getBoundingClientRect().top}px'
            ..onMouseDown.listen((_) {
  
negative.code:               if (!dragHappened)
                  tooltip.remove();
              });
  
          final String revisionLink = 'https://github.com/flutter/flutter/commit/${value.revision}';
  
          tooltip.setInnerHtml(
            '${value.value}$unit\n'
            'Flutter revision: <a href="$revisionLink" target="_blank">${value.revision}</a>\n'
            'Recorded on: ${new DateTime.fromMillisecondsSinceEpoch(value.createTimestamp)}\n'
            'Goal: $goal$unit\n'
            'Baseline: $baseline$unit',
            validator: const _NullValidator(),
          );
  
          final double left = bar.getBoundingClientRect().left;
          if (left < window.innerWidth / 2.0) {
            tooltip.style.left = '${bar.getBoundingClientRect().right + 5}px';
  