anchor.diff:      : super(key: key, children: children);
   }
   
  -class StackContainer extends OneChildListRenderObjectWrapper {
  +class StackContainer extends MultiChildRenderObjectWrapper {
     RenderStack root;
     RenderStack createNode() => new RenderStack();
   
  
positive.code: /*
   * RenderObjectWrappers correspond to a desired state of a RenderObject.
   * They are fully immutable, with one exception: A UINode which is a
   * Component which lives within an OneChildListRenderObjectWrapper's
   * children list, may be replaced with the "old" instance if it has
   * become stateful.
   */
  
negative.code: 
  final List<UINode> _emptyList = new List<UINode>();
  
  abstract class MultiChildRenderObjectWrapper extends RenderObjectWrapper {
  
    // In MultiChildRenderObjectWrapper subclasses, slots are RenderObject nodes
    // to use as the "insert before" sibling in ContainerRenderObjectMixin.add() calls
  
    final List<UINode> children;
  
    MultiChildRenderObjectWrapper({
      Object key,
      List<UINode> children
    }) : this.children = children == null ? _emptyList : children,
  