anchor.diff:   */
   class Router {
     final Map<String, _Route> _routes = new Map<String, _Route>();
  -  bool _useFragment;
  +  final bool useFragment;
  +  final Window _window;
     _Route _defaultRoute;
     _Route _currentRoute;
     String _lastTail = '';
  -  html.Window _window;
   
     /**
      * [useFragment] determines whether this Router uses pure paths with
  
positive.code: library route.client;
  
  import 'dart:async';
  import 'dart:html' as html;
  import 'package:logging/logging.dart';
  import 'url_pattern.dart';
  export 'url_pattern.dart';
  
negative.code:    * value is null which then determines the behavior based on
     * [History.supportsState].
     */
    Router({bool useFragment, Window windowImpl})
        : useFragment = (useFragment == null)
              ? !History.supportsState
              : useFragment,
          _window = (windowImpl == null) ? window : windowImpl;
  
    void addRoute({String name, Pattern path, bool defaultRoute: false,
        RouteHandler enter, RouteHandler leave, mount}) {
  