anchor.diff:  
   /// An overlay entry that is mimicking another widget.
   class MimicOverlayEntry {
  -  MimicOverlayEntry._(this._key) {
  +  MimicOverlayEntry._(this._handle) {
       _overlayEntry = new OverlayEntry(builder: _build);
  -    _initialGlobalBounds = _key.globalBounds;
  +    _initialGlobalBounds = _handle.globalBounds;
     }
   
     Rect _initialGlobalBounds;
   
  -  MimicableHandle _key;
  +  MimicableHandle _handle;
     OverlayEntry _overlayEntry;
   
     // Animation state
  
positive.code:       top: localPosition.y,
        width: globalBounds.width,
        height: globalBounds.height,
        child: new Mimic(original: _key)
      );
    }
  }
  
negative.code:   }
  
    void _stopMimic() {
      assert(_placeholderSize != null);
      if (mounted) {
        setState(() {
          _placeholderSize = null;
        });
      }
    }
  
    Widget build(BuildContext context) {
      if (_placeholderSize != null) {
        return new ConstrainedBox(
          constraints: new BoxConstraints.tight(_placeholderSize)
        );
      }
      return config.child;
    }
  }
  