anchor.diff:      if (oldComponent == null || !oldComponent._stateful)
         return false;
   
  +    assert(runtimeType == oldComponent.runtimeType);
       assert(key == oldComponent.key);
   
       // Make |this|, the newly-created object, into the "old" Component, and kill it
  
positive.code: 
  abstract class OneChildRenderObjectWrapper extends RenderObjectWrapper {
  
    OneChildRenderObjectWrapper({ Widget child, String key })
      : _child = child, super(key: key);
  
    Widget _child;
  
negative.code:     void advanceOldStartIndex() {
        oldStartIndex++;
        while (oldStartIndex < oldEndIndex &&
               oldNodeReordered(oldChildren[oldStartIndex].key)) {
          oldStartIndex++;
        }
      }
  