anchor.diff:      if (value == _rootNode) return;
   
       // Ensure that the root node has a size
  -    assert(value.size.width > 0);
  -    assert(value.size.height > 0);
  +    assert(_transformMode == SpriteBoxTransformMode.nativePoints
  +      || value.size.width > 0);
  +    assert(_transformMode == SpriteBoxTransformMode.nativePoints
  +      || value.size.height > 0);
   
       // Remove sprite box references
       if (_rootNode != null) _removeSpriteBoxReference(_rootNode);
  
positive.code:         rootNode.size = new Size(systemWidth, systemHeight);
          break;
        case SpriteBoxTransformMode.nativePoints:
          break;
        default:
          assert(false);
  
negative.code:     assert(rootNode != null);
      assert(rootNode._spriteBox == null);
  
      // Setup transform mode
      this.transformMode = mode;
  
      // Setup root node
      this.rootNode = rootNode;
    }
  
    void _removeSpriteBoxReference(Node node) {
  