[firebase_auth] Add a constructor for token result object with a token

packages/firebase_auth/lib/src/auth_result.dart

@@ -11,6 +11,12 @@ class AuthResult {
   AuthResult._(this._data, FirebaseApp app)
       : user = FirebaseUser._(_data['user'].cast<String, dynamic>(), app);
 
+  /// Constructs a authentication result from a user.
+  ///
+  /// This is useful when apps are implementing [FirebaseUser] and managing their
+  /// own auth mechanism.
+  AuthResult.fromUser(this.user) : _data = null;
+
   final Map<String, dynamic> _data;
 
   /// Returns the currently signed-in [FirebaseUser], or `null` if there isn't

packages/firebase_auth/lib/src/id_token_result.dart

@@ -15,6 +15,14 @@ class IdTokenResult {
   @visibleForTesting
   IdTokenResult(this._data, this._app);
 
+  /// Constructs a token result from a token.
+  ///
+  /// This is useful when apps are implementing [FirebaseUser] and managing their
+  /// own tokens.
+  IdTokenResult.fromToken(String token)
+      : _data = <String, String>{'token': token},
+        _app = null;
+
   final FirebaseApp _app;
 
   final Map<dynamic, dynamic> _data;

packages/firebase_auth/test/firebase_user_test.dart

@@ -0,0 +1,114 @@
+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+
+import 'package:firebase_auth/firebase_auth.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+final String testDisplayName = 'testDisplayName';
+final String testToken = 'testToken';
+
+/// Implements an app specific [FirebaseUser] object.
+///
+/// This is useful for making sure APIs used within the object are "implements"
+/// friendly with no private constructors.
+class TestFirebaseUser implements FirebaseUser {
+  String _username = 'test';
+  String _password = 'pwd';
+  String _email;
+  String _displayName = testDisplayName;
+  String _token = testToken;
+  String _uid;
+
+  @override
+  Future<void> delete() async => null;
+
+  @override
+  String get displayName => _displayName;
+
+  @override
+  String get email => _email;
+
+  @override
+  Future<IdTokenResult> getIdToken({bool refresh = false}) async {
+    return IdTokenResult.fromToken(_token);
+  }
+
+  @override
+  bool get isAnonymous => (_username != null);
+
+  @override
+  bool get isEmailVerified => (_password != null);
+
+  @override
+  Future<AuthResult> linkWithCredential(AuthCredential credential) async {
+    return AuthResult.fromUser(this);
+  }
+
+  @override
+  FirebaseUserMetadata get metadata => null;
+
+  @override
+  String get phoneNumber => 'None';
+
+  @override
+  String get photoUrl => 'None';
+
+  @override
+  List<UserInfo> get providerData => <UserInfo>[];
+
+  @override
+  String get providerId => 'None';
+
+  @override
+  Future<AuthResult> reauthenticateWithCredential(
+      AuthCredential credential) async {
+    return AuthResult.fromUser(this);
+  }
+
+  @override
+  Future<void> reload() async => null;
+
+  @override
+  Future<void> sendEmailVerification() async => null;
+
+  @override
+  String get uid => _uid;
+
+  @override
+  Future<void> unlinkFromProvider(String provider) async => null;
+
+  @override
+  Future<void> updateEmail(String email) async {
+    _email = email;
+    return null;
+  }
+
+  @override
+  Future<void> updatePassword(String password) async {
+    _password = password;
+    return null;
+  }
+
+  @override
+  Future<void> updatePhoneNumberCredential(AuthCredential credential) async =>
+      null;
+
+  @override
+  Future<void> updateProfile(UserUpdateInfo userUpdateInfo) async {
+    _displayName = userUpdateInfo.displayName;
+    return null;
+  }
+}
+
+void main() {
+  test('Expect TestFirebaseUser can be constructed and used.', () async {
+    final FirebaseUser testUser = TestFirebaseUser();
+    expect(testUser.displayName, testDisplayName);
+    expect(testUser.isAnonymous, false);
+    expect((await testUser.getIdToken()).token, testToken);
+    expect((await testUser.linkWithCredential(null)).user, testUser);
+  });
+}