anchor.diff:    }
   
   }
  -
  
positive.code: 
    RenderBlock({ List<RenderBox> children }) : super(children: children);
  
    void performLayout() {
      super.performLayout();
      // FIXME(eseidel): Block lays out its children with unconstrained height
      // yet itself remains constrained. Remember that our children wanted to
      // be taller than we are so we know to clip them (and not cause confusing
      // mismatch of painting vs. hittesting).
      _hasVisualOverflow = childrenHeight > size.height;
      size = constraints.constrain(new Size(constraints.maxWidth, childrenHeight));
      assert(!size.isInfinite);
    }
  
    void paint(PaintingCanvas canvas, Offset offset) {
  
negative.code: 
    // lays out RenderBox children in a vertical stack
    // uses the maximum width provided by the parent
  
    RenderBlockBase({
      List<RenderBox> children
  