anchor.diff:  class Router {
     final Map<UrlPattern, Handler> _handlers;
     final bool useFragment;
  +  bool _listen = false;
   
     /**
      * [useFragment] determines whether this Router uses pure paths with
  
positive.code:    * with the path version of the URL by converting the # to a /.
     */
    void handle(String path) {
      var url = _getUrl(path);
      if (url != null) {
        // always give handlers a non-fragment path
  
negative.code:   }
    
    void gotoPath(String path, String title) {
      _logger.finest('gotoPath $path');
      var url = _getUrl(path);
      if (url != null) {
        _go(path, title);
        // If useFragment, onHashChange will call handle for us.
        if (!_listen || !useFragment) {
          _handlers[url](path);
        }
      }
    }
  
  