anchor.diff:      : super(key: key, children: children);
   }
   
  -class StackContainer extends OneChildListRenderObjectWrapper {
  +class StackContainer extends MultiChildRenderObjectWrapper {
     RenderStack root;
     RenderStack createNode() => new RenderStack();
   
  
positive.code:     return false;
    }
  
    void syncRenderObject(OneChildListRenderObjectWrapper old) {
      super.syncRenderObject(old);
  
      if (root is! ContainerRenderObjectMixin)
  
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,
  