anchor.diff:      void advanceOldStartIndex() {
         oldStartIndex++;
         while (oldStartIndex < oldEndIndex &&
  -             oldNodeReordered(oldChildren[oldStartIndex]._key)) {
  +             oldNodeReordered(oldChildren[oldStartIndex].key)) {
           oldStartIndex++;
         }
       }
  
positive.code:       return oldNode;
      }
  
      if (oldNode != null && node._key != oldNode._key) {
        assert(oldNode.mounted);
        removeChild(oldNode);
        oldNode = null;
  
negative.code: 
  abstract class OneChildRenderObjectWrapper extends RenderObjectWrapper {
  
    OneChildRenderObjectWrapper({ String key, Widget child })
      : _child = child, super(key: key);
  
    Widget _child;
  