anchor.diff:  
   class _TooltipState extends State<Tooltip> {
   
  -  Performance _performance;
  +  AnimationController _controller;
     OverlayEntry _entry;
     Timer _timer;
   
     void initState() {
       super.initState();
  -    _performance = new Performance(duration: config.fadeDuration)
  +    _controller = new AnimationController(duration: config.fadeDuration)
         ..addStatusListener((PerformanceStatus status) {
           switch (status) {
             case PerformanceStatus.completed:
  
positive.code:   }
  
    void resetShowTimer() {
      assert(_performance.status == PerformanceStatus.completed);
      assert(_entry != null);
      _timer = new Timer(config.showDuration, hideTooltip);
    }
  
negative.code:   void didUpdateConfig(Tooltip oldConfig) {
      super.didUpdateConfig(oldConfig);
      if (config.fadeDuration != oldConfig.fadeDuration)
        _controller.duration = config.fadeDuration;
      if (_entry != null &&
          (config.message != oldConfig.message ||
           config.backgroundColor != oldConfig.backgroundColor ||
  