anchor.diff:    }
   
     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);
  +    if (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 }) {
  
positive.code:   RenderBox _root;
    RenderBox get root => _root;
    void set root (RenderBox value) {
      assert(value != null);
      _root = value;
      adoptChild(_root);
      markNeedsLayout();
    }
  
  
negative.code: // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file.
  
  import 'node.dart';
  import 'dart:sky' as sky;
  
  // ABSTRACT LAYOUT
  