anchor.diff:    final Window _window;
     _Route _defaultRoute;
     _Route _currentRoute;
  -  String _lastTail = '';
  +  String _lastHead = '';
   
     /**
      * [useFragment] determines whether this Router uses pure paths with
  
positive.code:   }
  
    Future<bool> _processNewRoute(String path, UrlMatch match, _Route route,
                                  String prefix) {
      var event = new RouteEvent(match.match, match.parameters);
      // before we make this a new current route, leave the old
      return _leaveCurrentRoute(event).then((bool allowNavigation) {
  
negative.code:   }
  
    _Route({this.path, this.enter, this.leave, this.child});
  }
  
  class RouteEvent {
  