How to use dart Future in js?

Question

this is my sample programe:

import 'dart:html';
import 'package:js/js.dart';
import 'package:js/js_util.dart';

void main() {
  setProperty(window, 'login', allowInterop(login));
}

Future<String> login (String username,String password) async {
  return 'test';
}

I wanna know when I call login in JavaScript how to resolve the Future < String> ?