1. override
Future<Response> handle(Request request)

Source 

@override Future<Response> handle(Request request) async {
  if (request.url.path.endsWith('/')) {
    final url = request.handlerPath + request.url.path;
    return redirectPermanently('/' + url.split('/').where((s) => s.isNotEmpty).join('/'));
  }
  return await super.handle(request);
}