anchor.diff:    double getMaxIntrinsicHeight(BoxConstraints constraints) {
       return _getIntrinsicHeight(constraints);
     }
  -
  -  double computeDistanceToActualBaseline(TextBaseline baseline) {
  -    return defaultComputeDistanceToFirstActualBaseline(baseline);
  -  }
  -
  -  double _childrenHeight;
  -  double get childrenHeight => _childrenHeight;
  -
  -  void markNeedsLayout() {
  -    _childrenHeight = null;
  -    super.markNeedsLayout();
  -  }
  -
  -  void performLayout() {
  -    assert(constraints is BoxConstraints);
  -    double width = constraints.constrainWidth(constraints.maxWidth);
  -    BoxConstraints innerConstraints = _getInnerConstraintsForWidth(width);
  -    double y = 0.0;
  -    RenderBox child = firstChild;
  -    while (child != null) {
  -      child.layout(innerConstraints, parentUsesSize: true);
  -      assert(child.parentData is BlockParentData);
  -      child.parentData.position = new Point(0.0, y);
  -      y += child.size.height;
  -      child = child.parentData.nextSibling;
  -    }
  -    _childrenHeight = y;
  -  }
  -
  -  void hitTestChildren(HitTestResult result, { Point position }) {
  -    defaultHitTestChildren(result, position: position);
  -  }
  -
  -  void paint(PaintingCanvas canvas, Offset offset) {
  -    defaultPaint(canvas, offset);
  -  }
  -
  -}
  -
  -class RenderBlock extends RenderBlockBase {
  -
  -  // sizes itself to the height of its child stack
  -
  -  RenderBlock({ List<RenderBox> children }) : super(children: children);
   
     bool _hasVisualOverflow = false;
   
  
positive.code:       markNeedsLayout();
    }
  
    bool get sizedByParent => true;
  
    void performResize() {
  
negative.code:     return width;
    }
  
    double _getIntrinsicHeight(BoxConstraints constraints) {
      double height = 0.0;
      double width = constraints.constrainWidth(constraints.maxWidth);
      BoxConstraints innerConstraints = new BoxConstraints.tightFor(width: width);
      RenderBox child = firstChild;
      while (child != null) {
        double childHeight = child.getMinIntrinsicHeight(innerConstraints);
  