anchor.diff:  
     String get basePath => _basePath;
     void set basePath(String bp) {
  -    if (_routes.length > 0) {
  -      throw new _RouterException("Cannot alter basePath after adding routes.");
  +    if (routes.length > 0) {
  +      throw new RouterException("Cannot alter basePath after adding routes.");
       }
       _basePath = bp;
     }
  
positive.code:     return _createRoute(pattern).handler;
    }
  
    _ResourceRoute _createRoute(String pattern) {
      if (basePath != null) {
        pattern = basePath + pattern;
      }
  
negative.code: 
    /// Creates a new [Router].
    Router() {
      routes = [];
      unhandledRequestHandler = _handleUnhandledRequest;
    }
  
  