Add google_sign_in_web plugin.

packages/google_sign_in/google_sign_in_web/CHANGELOG.md

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+## 1.0.0
+
+* Flutter for web initial release

packages/google_sign_in/google_sign_in_web/LICENSE

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+Copyright 2016, the Flutter project authors. All rights reserved.
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+    * Neither the name of Google Inc. nor the names of its
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

packages/google_sign_in/google_sign_in_web/README.md

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+# google_sign_in_web
+
+The web implementation of [google_sign_in](https://pub.dev/google_sign_in/google_sign_in)
+
+## Web integration
+
+First, go through the instructions [here](https://developers.google.com/identity/sign-in/web/sign-in#before_you_begin) to create your Google Sign-In OAuth client ID.
+
+On your `web/index.html` file, add the following `meta` tag, somewhere in the
+`head` of the document:
+
+```
+  <meta name="google-signin-client_id" content="YOUR_GOOGLE_SIGN_IN_OAUTH_CLIENT_ID.apps.googleusercontent.com">
+```
+
+Read the rest of the instructions if you need to add extra APIs (like Google People API).
+
+
+## Usage
+
+### Import the package
+To use this plugin, follow the [plugin installation instructions](https://pub.dartlang.org/packages/google_sign_in#pub-pkg-tab-installing).
+
+### Use the plugin
+Add the following import to your Dart code:
+
+```dart
+import 'package:google_sign_in/google_sign_in.dart';
+```
+
+Initialize GoogleSignIn with the scopes you want:
+
+```dart
+GoogleSignIn _googleSignIn = GoogleSignIn(
+  scopes: [
+    'email',
+    'https://www.googleapis.com/auth/contacts.readonly',
+  ],
+);
+```
+[Full list of available scopes](https://developers.google.com/identity/protocols/googlescopes).
+
+You can now use the `GoogleSignIn` class to authenticate in your Dart code, e.g.
+
+```dart
+Future<void> _handleSignIn() async {
+  try {
+    await _googleSignIn.signIn();
+  } catch (error) {
+    print(error);
+  }
+}
+```
+
+## Example
+
+Find the example wiring in the [Google sign-in example application](https://github.com/flutter/plugins/blob/master/packages/google_sign_in/google_sign_in/example/lib/main.dart).
+
+## API details
+
+See the [google_sign_in.dart](https://github.com/flutter/plugins/blob/master/packages/google_sign_in/google_sign_in/lib/google_sign_in.dart) for more API details.
+
+## Issues and feedback
+
+Please file [issues](https://github.com/flutter/flutter/issues/new)
+to send feedback or report a bug. Thank you!

packages/google_sign_in/google_sign_in_web/lib/google_sign_in_web.dart

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+// 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 'dart:html' as html;
+
+import 'package:google_sign_in_platform_interface/google_sign_in_platform_interface.dart';
+import 'package:flutter_web_plugins/flutter_web_plugins.dart';
+import 'package:js/js.dart';
+import 'package:meta/meta.dart';
+
+import 'src/generated/gapiauth2.dart' as auth2;
+// TODO: Remove once this lands https://github.com/dart-lang/language/issues/671
+import 'src/generated/gapiauth2.dart' show GoogleAuthExtensions;
+import 'src/load_gapi.dart' as gapi;
+import 'src/utils.dart' show gapiUserToPluginUserData;
+
+const String _kClientIdMetaSelector = 'meta[name=google-signin-client_id]';
+const String _kClientIdAttributeName = 'content';
+
+@visibleForTesting
+String gapiUrl = 'https://apis.google.com/js/platform.js';
+
+/// Implementation of the google_sign_in plugin for Web
+class GoogleSignInPlugin extends GoogleSignInPlatform {
+  GoogleSignInPlugin() {
+    _autoDetectedClientId = html
+        .querySelector(_kClientIdMetaSelector)
+        ?.getAttribute(_kClientIdAttributeName);
+
+    _isGapiInitialized = gapi.inject(gapiUrl).then((_) => gapi.init()).then((_) {
+      isInitialized = true;
+    });
+  }
+
+  Future<void> _isGapiInitialized;
+
+  @visibleForTesting
+  Future<void> get isInitializing => _isGapiInitialized;
+  @visibleForTesting
+  bool isInitialized;
+
+  String _autoDetectedClientId;
+  FutureOr<auth2.GoogleUser> _lastSeenUser;
+
+  static void registerWith(Registrar registrar) {
+    GoogleSignInPlatform.instance = GoogleSignInPlugin();
+  }
+
+  @override
+  Future<void> init(
+      {@required String hostedDomain,
+      List<String> scopes = const <String>[],
+      SignInOption signInOption = SignInOption.standard,
+      String clientId}) async {
+    final String appClientId = clientId ?? _autoDetectedClientId;
+    assert(
+        appClientId != null,
+        'ClientID not set. Either set it on a '
+        '<meta name="google-signin-client_id" content="CLIENT_ID" /> tag,'
+        ' or pass clientId when calling init()');
+
+    await _isGapiInitialized;
+
+    // This init returns an  user, so let's wait for its future
+    final auth2.GoogleAuth auth = auth2.init(auth2.ClientConfig(
+      hosted_domain: hostedDomain,
+      // The js lib wants a space-separated list of values
+      scope: scopes.join(' '),
+      client_id: appClientId,
+    ));
+
+    // Subscribe to changes in the auth user, and cache the latest seen for signInSilently
+    final Completer<auth2.GoogleUser> initUserCompleter =
+        Completer<auth2.GoogleUser>();
+
+    auth.currentUser.listen(allowInterop((auth2.GoogleUser nextUser) {
+      if (!initUserCompleter.isCompleted) {
+        initUserCompleter.complete(nextUser);
+      } else {
+        _lastSeenUser = nextUser;
+      }
+    }));
+    _lastSeenUser = initUserCompleter.future;
+
+    return null;
+  }
+
+  @override
+  Future<GoogleSignInUserData> signInSilently() async {
+    await _isGapiInitialized;
+
+    return gapiUserToPluginUserData(await _lastSeenUser);
+  }
+
+  @override
+  Future<GoogleSignInUserData> signIn() async {
+    await _isGapiInitialized;
+
+    return gapiUserToPluginUserData(await auth2.getAuthInstance().signIn());
+  }
+
+  @override
+  Future<GoogleSignInTokenData> getTokens(
+      {@required String email, bool shouldRecoverAuth}) async {
+    await _isGapiInitialized;
+
+    final auth2.GoogleUser currentUser =
+        auth2.getAuthInstance()?.currentUser?.get();
+    final auth2.AuthResponse response = currentUser.getAuthResponse();
+
+    return GoogleSignInTokenData(
+        idToken: response.id_token, accessToken: response.access_token);
+  }
+
+  @override
+  Future<void> signOut() async {
+    await _isGapiInitialized;
+
+    return auth2.getAuthInstance().signOut();
+  }
+
+  @override
+  Future<void> disconnect() async {
+    await _isGapiInitialized;
+
+    final auth2.GoogleUser currentUser =
+        auth2.getAuthInstance()?.currentUser?.get();
+    return currentUser.disconnect();
+  }
+
+  @override
+  Future<bool> isSignedIn() async {
+    await _isGapiInitialized;
+
+    final auth2.GoogleUser currentUser =
+        auth2.getAuthInstance()?.currentUser?.get();
+    return currentUser.isSignedIn();
+  }
+
+  @override
+  Future<void> clearAuthCache({String token}) async {
+    await _isGapiInitialized;
+
+    _lastSeenUser = null;
+    return auth2.getAuthInstance().disconnect();
+  }
+}