abstract auth manager with conditionall imports - for medium article

auth_manager.dart

import 'auth0_manager_stub.dart'
    if (dart.library.io) 'auth0_manager.dart'
    if (dart.library.js) 'auth0_manager_for_web.dart';

abstract class AuthManager {
  static AuthManager _instance;

  static AuthManager get instance {
    _instance ??= getManager();
    return _instance;
  }

  Future<User> login([Map<String, String> authResponse]);
  Future<User> userFromTokens(String idToken, String accessToken);
  Future<void> logout();
}