anchor.diff:      });
   
       // If we still have time left, go on.
  -    if (left > 0)
  +    if (left > 0) {
         window.requestAnimationFrame(_advance);
  -    else
  +    } else {
         onComplete();
  +    }
   
       // TODO: Fire a "step" event!
     }
  -}+}
  
positive.code:    * Advances the animation by one step.
     */
    _advance(num highResTime) {
      if (_paused || _stopped)
        return;
  
      var currentTime = new Date.now().millisecondsSinceEpoch;
  
  
negative.code: part of animation;
  
  class StyleAnimation extends Animation {
    final Map<String, Object> fromProperties = {};
    final Map<String, Object> currentProperties = {};
    final Map<String, String> units = {};
  
    final Element element;
  
    Map<String, Object> toProperties = {};
  
    /**
     * Properties are set asynchronously (due to getComputedStyle()).
  