anchor.diff:        RenderObject debugPreviousActiveLayout;
         assert(!_debugMutationsLocked);
         assert(!_debugDoingThisLayoutWithCallback);
  +      assert(_debugCanParentUseSize != null);
         assert(() {
           _debugMutationsLocked = true;
  -        _debugCanParentUseSize = false;
           _debugDoingThisLayout = true;
           debugPreviousActiveLayout = _debugActiveLayout;
           _debugActiveLayout = this;
  
positive.code:     assert(() {
        _debugActiveLayout = debugPreviousActiveLayout;
        _debugDoingThisLayout = false;
        _debugCanParentUseSize = null;
        _debugMutationsLocked = false;
        return true;
      });
  
negative.code:     assert(parent == null);
      assert(_relayoutSubtreeRoot == null);
      _relayoutSubtreeRoot = this;
      assert(() {
        _debugCanParentUseSize = false;
        return true;
      });
      _nodesNeedingLayout.add(this);
      _nodesNeedingPaint.add(this);
      scheduler.ensureVisualUpdate();
  