anchor.diff:    static final List<String> _progress = <String>['-', r'\', '|', r'/', '-', r'\', '|', '/'];
   
     final String message;
  +  final _FinishCallback onFinish;
     Stopwatch stopwatch;
     Timer timer;
     int index = 1;
  
positive.code: 
    @override
    void stop({ bool showElapsedTime: true }) {
      if (!live)
        return;
      live = false;
  
negative.code: 
    @override
    void cancel() {
      onFinish();
  
      if (!live)
        return;
      live = false;
  