anchor.diff:    void debugFillProperties(DiagnosticPropertiesBuilder properties) {
       super.debugFillProperties(properties);
       properties.add(IntProperty('duration', duration.inMilliseconds, unit: 'ms'));
  -    properties.add(IntProperty('reverseDuration', reverseDuration?.inMilliseconds, unit: 'ms', defaultValue: null));
     }
   }
   
  
positive.code:     this.animateShadowColor = true,
      Curve curve = Curves.linear,
      @required Duration duration,
      Duration reverseDuration,
    }) : assert(child != null),
         assert(shape != null),
         assert(clipBehavior != null),
  
negative.code:   /// The duration over which to animate the parameters of this container.
    final Duration duration;
  
    @override
    ImplicitlyAnimatedWidgetState<ImplicitlyAnimatedWidget> createState();
  
  