why could not import the Result class from dart async

I want to import the Result from dart async, but after I import the dart async, the Result still could not be found. This is my classs:

import 'package:reddwarf_dict/models/login/cellphone_existence_check.dart';
import 'dart:async';

class LoginApi {
  static Future<Result<CellphoneExistenceCheck>> checkPhoneExist(
      String phone, String countryCode) async {
    return Result.value(null);
  }
}

this is the message that tell me could not import the Result:

The name 'Result' isn't a type so it can't be used as a type argument.

what should I to to import the Result success? The Dart environment SDK like this:

environment:
  sdk: ">=2.7.0 <3.0.0"

>Solution :

You need to add the async package to your Flutter project: flutter pub add async

Or with dart:
dart pub add async

And in your code you need this import:

import 'package:async/async.dart';