anchor.diff:        if (_performance.isCompleted)
           _maybeCallOnDismissed();
         else if (!_performance.isAnimating)
  -        _performance.progress = 0.0;
  +        _performance.reverse();
       });
     }
   
  
positive.code:   }
  
    void _maybeCallOnDismissed() {
      if (onDismissed != null)
        onDismissed();
    }
  
negative.code: import 'package:sky/widgets/widget.dart';
  import 'package:vector_math/vector_math.dart';
  
  const Duration _kCardDismissFadeout = const Duration(milliseconds: 200);
  const double _kMinFlingVelocity = 700.0;
  const double _kMinFlingVelocityDelta = 400.0;
  const double _kFlingVelocityScale = 1.0/300.0;
  