anchor.diff:    RenderBox _root;
     RenderBox get root => _root;
     void set root (RenderBox value) {
  -    assert(value != null);
  +    if (_root != null)
  +      dropChild(_root);
       _root = value;
  -    adoptChild(_root);
  +    if (_root != null)
  +      adoptChild(_root);
       markNeedsLayout();
     }
   
  
positive.code:   }
  
    void relayout() {
      assert(root != null);
      root.layout(new BoxConstraints(
          minWidth: width, maxWidth: width, minHeight: height, maxHeight: height));
      assert(root.width == width);
      assert(root.height == height);
    }
  
    void rotate({ int oldAngle, int newAngle, Duration time }) {
  
negative.code:     defaultPaint(canvas);
    }
  }
  