anchor.diff:    }
   
     void _maybeCallOnDismissed() {
  +    _performance.stop();
  +    _performance.removeListener(_handleAnimationProgressChanged);
       if (onDismissed != null)
         onDismissed();
     }
  
positive.code: import 'package:sky/widgets/widget.dart';
  import 'package:vector_math/vector_math.dart';
  
  const Duration _kCardDismissFadeout = const Duration(milliseconds: 300);
  const double _kMinFlingVelocity = 700.0;
  const double _kMinFlingVelocityDelta = 400.0;
  const double _kFlingVelocityScale = 1.0/300.0;
  
negative.code:       if (_performance.isCompleted)
          _maybeCallOnDismissed();
        else if (!_performance.isAnimating)
          _performance.reverse();
      });
    }
  
  