anchor.diff:    void remove() {
       RenderObjectWrapper ancestor = findAncestorRenderObjectWrapper();
       assert(ancestor is RenderObjectWrapper);
  +    assert(ancestor.root == root.parent);
       ancestor.detachChildRoot(this);
       super.remove();
     }
  -
  -}
  +}
  
positive.code:     });
    }
  
    void syncFields(FutureImage source) {
      bool needToResolveImage = (image != source.image);
      image = source.image;
  
negative.code:   ClipOval({ String key, Widget child })
      : super(key: key, child: child);
  
    RenderClipOval createNode() => new RenderClipOval();
    RenderClipOval get root => super.root;
  
    // Nothing to sync, so we don't implement syncRenderObject()
  }
  
  
  // POSITIONING AND SIZING NODES
  
  class Transform extends OneChildRenderObjectWrapper {
    Transform({ String key, this.transform, Widget child })
      : super(key: key, child: child);
  
    final Matrix4 transform;
  
    RenderTransform createNode() => new RenderTransform(transform: transform);
    RenderTransform get root => super.root;
  
    void syncRenderObject(Transform old) {
      super.syncRenderObject(old);
      root.transform = transform;
    }
  }
  
  class Padding extends OneChildRenderObjectWrapper {
    Padding({ String key, this.padding, Widget child })
      : super(key: key, child: child);
  
    final EdgeDims padding;
  
    RenderPadding createNode() => new RenderPadding(padding: padding);
    RenderPadding get root => super.root;
  
    void syncRenderObject(Padding old) {
      super.syncRenderObject(old);
      root.padding = padding;
    }
  }
  
  class Center extends OneChildRenderObjectWrapper {
    Center({ String key, Widget child })
      : super(key: key, child: child);
  
    RenderPositionedBox createNode() => new RenderPositionedBox();
    RenderPositionedBox get root => super.root;
  
    // Nothing to sync, so we don't implement syncRenderObject()
  }
  
  class SizedBox extends OneChildRenderObjectWrapper {
    SizedBox({ String key, this.width, this.height, Widget child })
      : super(key: key, child: child);
  
    final double width;
    final double height;
  
    RenderConstrainedBox createNode() => new RenderConstrainedBox(additionalConstraints: _additionalConstraints());
    RenderConstrainedBox get root => super.root;
  
    BoxConstraints _additionalConstraints() {
      BoxConstraints result = const BoxConstraints();
      if (width != null)
        result = result.applyWidth(width);
      if (height != null)
  