anchor.diff:     * [History.supportsState].
      */
     Router({bool useFragment})
  -      : _handlers = new Map<UrlPattern, Handler>(),
  +      : _handlers = new LinkedHashMap<UrlPattern, Handler>(),
           useFragment = (useFragment == null)
               ? !History.supportsState
               : useFragment;
   
  +  /**
  +   * Registers a function that will be invoked when the router handles a URL
  +   * that matches [pattern].
  +   */
     void addHandler(UrlPattern pattern, Handler handler) {
       _logger.finest('addHandler $pattern');
       _handlers[pattern] = handler;
  
positive.code:       window.history.pushState(null, title, path);
      }
    }
    
    /**
     * Returns an [Event] handler suitable for use as a click handler on [:<a>:]
     * elements. The handler reverses [ur] with [args] and uses [window.pushState]
  
negative.code:       throw new ArgumentError('Unknown URL pattern: $url');
      }
    }
  
    void gotoPath(String path, String title) {
      _logger.finest('gotoPath $path');
      var url = _getUrl(path);
  