anchor.diff:        }
       }
   
  -    assert(!node.mounted);
  -    node.setParent(this);
  -    node._sync(oldNode, slot);
  -    assert(node.root is RenderObject);
  -    return node;
  +    assert(!newNode.mounted);
  +    newNode.setParent(this);
  +    newNode._sync(oldNode, slot);
  +    assert(newNode.root is RenderObject);
  +    return newNode;
     }
   
     String toString() {
  
positive.code:     }
  
      if (oldNode != null) {
        if (oldNode.runtimeType == node.runtimeType && oldNode.key == node.key) {
          if (node._retainStatefulNodeIfPossible(oldNode)) {
            assert(oldNode.mounted);
            assert(!node.mounted);
            oldNode.setParent(this);
            oldNode._sync(node, slot);
            assert(oldNode.root is RenderObject);
            return oldNode;
          }
  
negative.code:   //      assert(_built != null && old == null)
    // 3) Syncing against an old version
    //      assert(_built == null && old != null)
    void _sync(Component old, dynamic slot) {
      assert(_built == null || old == null);
      assert(!_disqualifiedFromEverAppearingAgain);
  
      _slot = slot;
  
      var oldBuilt;
      if (old == null) {
        oldBuilt = _built;
      } else {
        assert(_built == null);
        oldBuilt = old._built;
      }
  
      int lastOrder = _currentOrder;
  