anchor.diff:  
   import 'package:sky/mojo/activity.dart' as activity;
   
  +import '../base/debug.dart';
   import '../base/hit_test.dart';
   import '../rendering/box.dart';
   import '../rendering/object.dart';
  
positive.code: 
      if (oldNode != null) {
        if (oldNode.runtimeType == newNode.runtimeType && oldNode.key == newNode.key) {
          if (newNode._retainStatefulNodeIfPossible(oldNode)) {
            assert(oldNode.mounted);
            assert(!newNode.mounted);
            oldNode.setParent(this);
  
negative.code:   RenderObject get root => _root;
  
    // Subclasses which implements Nodes that become stateful may return true
    // if the node has become stateful and should be retained.
    // This is called immediately before _sync().
    // Component.retainStatefulNodeIfPossible() calls syncFields().
    bool retainStatefulNodeIfPossible(Widget newNode) => false;
  
    void _sync(Widget old, dynamic slot);
    void updateSlot(dynamic newSlot);
  