anchor.diff:      _root = null;
     }
   
  +  int _nodeDepth;
     void _ensureDepth() {
       if (_nodeDepth == null) {
  -      _nodeDepth = 0;
  -      UINode parent = _parent;
  -      while (parent != null) {
  -        _nodeDepth++;
  -        parent = parent._parent;
  -      }
  +      _parent.ensureDepth();
  +      _nodeDepth = _parent._nodeDepth + 1;
       }
     }
   
  
positive.code:   UINode _parent;
    sky.Node _root;
    bool _defunct = false;
    int _nodeDepth;
  
    UINode({ Object key }) {
      _key = key == null ? "$runtimeType" : "$runtimeType-$key";
  
negative.code:       return;
  
      _ensureDepth();
      print((' ' * _nodeDepth) + message);
    }
  
    void _traceSync(_SyncOperation op, String key) {
  