anchor.diff:  import 'overlay.dart';
   
   abstract class Route<T> {
  +  /// The navigator that the route is in, if any.
  +  NavigatorState get navigator => _navigator;
  +  NavigatorState _navigator;
  +
     List<OverlayEntry> get overlayEntries;
     void didPush(OverlayState overlay, OverlayEntry insertionPoint) { }
     void didPop(T result) { }
  
positive.code: typedef Route RouteFactory(NamedRouteSettings settings);
  
  class NavigatorObserver {
    NavigatorState _navigator;
    NavigatorState get navigator => _navigator;
    void didPush(Route route, Route previousRoute) { }
    void didPop(Route route, Route previousRoute) { }
  }
  
negative.code:     return null;
    }
  
    bool _debugLocked = false; // used to prevent re-entrant calls to push, pop, and friends
  
    void pushNamed(String name, { Set<Key> mostValuableKeys }) {
      assert(!_debugLocked);
      assert(name != null);
      NamedRouteSettings settings = new NamedRouteSettings(
        name: name,
  