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