anchor.diff:    Widget build() {
       List<Widget> layers = new List<Widget>();
   
  -    if (children != null)
  +    if (children != null) {
         layers.addAll(children);
  +    }
   
       if (_activeOverlay != null) {
         layers.add(
  
positive.code:     this.children,
      this.overlay,
      this.duration: const Duration(milliseconds: 200),
      this.curve: linear
    }) : super(key: key);
  
    List<Widget> children;
    GlobalKey overlay;
    Duration duration;
    Curve curve;
  
    void syncFields(MimicOverlay source) {
      children = source.children;
  
negative.code:       );
      }
  
      return new Stack(layers);
    }
  }
  