anchor.diff:    stop() {
       super.stop();
   
  -    currentProperties = {};
  +    currentProperties.clear();
   
       _setFromProperties();
   
  
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()).
  