anchor.diff:    stop() {
       super.stop();
   
  -    currentProperties = {};
  +    currentProperties.clear();
   
       _setFromProperties();
   
  
positive.code:     });
  
      // If we still have time left, go on.
      if (left > 0)
        window.requestAnimationFrame(_advance);
      else
        onComplete();
  
      // TODO: Fire a "step" event!
    }
  }
negative.code:    * Advances the animation by one step.
     */
    _advance(num highResTime) {
      if (_paused || _stopped) {
        return;
      }
  
      var currentTime = new Date.now().millisecondsSinceEpoch;
  
  