anchor.diff:      assert(attached);
       assert(parent is! RenderObject);
       assert(!owner._debugDoingLayout);
  -    assert(_relayoutSubtreeRoot == null);
  -    _relayoutSubtreeRoot = this;
  +    assert(_relayoutBoundary == null);
  +    _relayoutBoundary = this;
       assert(() {
         _debugCanParentUseSize = false;
         return true;
  
positive.code:   @override
    String toString() {
      String header = '$runtimeType';
      if (_relayoutSubtreeRoot != null && _relayoutSubtreeRoot != this) {
        int count = 1;
        RenderObject target = parent;
        while (target != null && target != _relayoutSubtreeRoot) {
          target = target.parent;
          count += 1;
        }
        header += ' relayoutSubtreeRoot=up$count';
      }
      if (_needsLayout)
        header += ' NEEDS-LAYOUT';
  
negative.code:   }
  
    void _layoutWithoutResize() {
      assert(_relayoutBoundary == this);
      RenderObject debugPreviousActiveLayout;
      assert(!_debugMutationsLocked);
      assert(!_doingThisLayoutWithCallback);
  