anchor.diff:    }
   }
   
  -class FlexContainer extends OneChildListRenderObjectWrapper {
  +class FlexContainer extends MultiChildRenderObjectWrapper {
     RenderFlex root;
     RenderFlex createNode() => new RenderFlex(direction: this.direction);
   
  
positive.code: 
  final List<UINode> _emptyList = new List<UINode>();
  
  abstract class OneChildListRenderObjectWrapper extends RenderObjectWrapper {
  
    // In OneChildListRenderObjectWrapper subclasses, slots are RenderObject nodes
    // to use as the "insert before" sibling in ContainerRenderObjectMixin.add() calls
  
    final List<UINode> children;
  
    OneChildListRenderObjectWrapper({
      Object key,
      List<UINode> children
    }) : this.children = children == null ? _emptyList : children,
  
negative.code:     : super(key: key, children: children);
  }
  
  class StackContainer extends MultiChildRenderObjectWrapper {
    RenderStack root;
    RenderStack createNode() => new RenderStack();
  
  