[google_sign_in] Bridge google_sign_in and googleapis. (#2824)

Introduce the first version of the `extension_google_sign_in_as_googleapis_auth` package.

This package lets users of the `google_sign_in` plugin to create an authenticated HTTP clients (from `googleapis_auth`) that can access any APIs from the `googleapis` package.

.gitignore

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+# Miscellaneous
+*.class
+*.log
+*.pyc
+*.swp
+.DS_Store
+.atom/
+.buildlog/
+.history
+.svn/
+
+# IntelliJ related
+*.iml
+*.ipr
+*.iws
+.idea/
+
+# The .vscode folder contains launch configuration and tasks you configure in
+# VS Code which you may wish to be included in version control, so this line
+# is commented out by default.
+#.vscode/
+
+# Flutter/Dart/Pub related
+**/doc/api/
+.dart_tool/
+.flutter-plugins
+.flutter-plugins-dependencies
+.packages
+.pub-cache/
+.pub/
+build/
+
+# Android related
+**/android/**/gradle-wrapper.jar
+**/android/.gradle
+**/android/captures/
+**/android/gradlew
+**/android/gradlew.bat
+**/android/local.properties
+**/android/**/GeneratedPluginRegistrant.java
+
+# iOS/XCode related
+**/ios/**/*.mode1v3
+**/ios/**/*.mode2v3
+**/ios/**/*.moved-aside
+**/ios/**/*.pbxuser
+**/ios/**/*.perspectivev3
+**/ios/**/*sync/
+**/ios/**/.sconsign.dblite
+**/ios/**/.tags*
+**/ios/**/.vagrant/
+**/ios/**/DerivedData/
+**/ios/**/Icon?
+**/ios/**/Pods/
+**/ios/**/.symlinks/
+**/ios/**/profile
+**/ios/**/xcuserdata
+**/ios/.generated/
+**/ios/Flutter/App.framework
+**/ios/Flutter/Flutter.framework
+**/ios/Flutter/Flutter.podspec
+**/ios/Flutter/Generated.xcconfig
+**/ios/Flutter/app.flx
+**/ios/Flutter/app.zip
+**/ios/Flutter/flutter_assets/
+**/ios/Flutter/flutter_export_environment.sh
+**/ios/ServiceDefinitions.json
+**/ios/Runner/GeneratedPluginRegistrant.*
+
+# Exceptions to above rules.
+!**/ios/**/default.mode1v3
+!**/ios/**/default.mode2v3
+!**/ios/**/default.pbxuser
+!**/ios/**/default.perspectivev3
+!/packages/flutter_tools/test/data/dart_dependencies_test/**/.packages

.metadata

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+# This file tracks properties of this Flutter project.
+# Used by Flutter tool to assess capabilities and perform upgrades etc.
+#
+# This file should be version controlled and should not be manually edited.
+
+version:
+  revision: 5e3e5a2a1a977c34b22f3709109fd237b5cab9c6
+  channel: master
+
+project_type: package

CHANGELOG.md

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+## 1.0.0
+
+* First published version.

LICENSE

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+Copyright 2020 The Flutter 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 LLC 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.

README.md

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+# extension_google_sign_in_as_googleapis_auth
+
+A bridge package between Flutter's [`google_sign_in` plugin](https://pub.dev/packages/google_sign_in) and Dart's [`googleapis` package](https://pub.dev/packages/googleapis), that is able to create [`googleapis_auth`-like `AuthClient` instances](https://pub.dev/documentation/googleapis_auth/latest/googleapis_auth.auth/AuthClient-class.html) directly from the `GoogleSignIn` plugin.
+
+## Usage
+
+This package is implemented as an [extension method](https://dart.dev/guides/language/extension-methods) on top of the `GoogleSignIn` plugin.
+
+In order to use it, you need to add a `dependency` to your `pubspec.yaml`. Then, wherever you're importing `package:google_sign_in/google_sign_in.dart`, add the following:
+
+```dart
+...
+import 'package:extension_google_sign_in_as_googleapis_auth/extension_google_sign_in_as_googleapis_auth.dart';
+...
+```
+
+From that moment on, your `GoogleSignIn` instance will have an additional `Future<AuthClient> authenticatedClient()` method that you can call once your sign in is successful to retrieve an `AuthClient`.
+
+That object can then be used to create instances of `googleapis` API clients:
+
+```dart
+...
+final peopleApi = PeopleApi(await _googleSignIn.authenticatedClient());
+final response = await peopleApi.people.connections.list(
+  'people/me',
+  personFields: 'names',
+);
+...
+```
+
+## Example
+
+This package contains a modified version of Flutter's Google Sign In example app that uses `package:googleapis`' API clients, instead of raw http requests.
+
+See it [here](https://github.com/flutter/plugins/blob/master/packages/google_sign_in/extension_google_sign_in_as_googleapis_auth/example/lib/main.dart).
+
+The original code (and its license) can be seen [here](https://github.com/flutter/plugins/tree/master/packages/google_sign_in/google_sign_in/example/lib/main.dart).
+
+## Testing
+
+Run tests with `flutter test`.
+
+## Issues and feedback
+
+Please file [issues](https://github.com/flutter/flutter/issues/new)
+to send feedback or report a bug. Thank you!

example/README.md

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+# extension_google_sign_in_example
+
+Demonstrates how to use the google_sign_in plugin with the `googleapis` package.
+
+## Getting Started
+
+For help getting started with Flutter, view our online
+[documentation](http://flutter.io/).

example/android/app/build.gradle

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+def localProperties = new Properties()
+def localPropertiesFile = rootProject.file('local.properties')
+if (localPropertiesFile.exists()) {
+    localPropertiesFile.withReader('UTF-8') { reader ->
+        localProperties.load(reader)
+    }
+}
+
+def flutterRoot = localProperties.getProperty('flutter.sdk')
+if (flutterRoot == null) {
+    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
+}
+
+def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
+if (flutterVersionCode == null) {
+    flutterVersionCode = '1'
+}
+
+def flutterVersionName = localProperties.getProperty('flutter.versionName')
+if (flutterVersionName == null) {
+    flutterVersionName = '1.0'
+}
+
+apply plugin: 'com.android.application'
+apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
+
+android {
+    compileSdkVersion 28
+
+    lintOptions {
+        disable 'InvalidPackage'
+    }
+
+    defaultConfig {
+        applicationId "io.flutter.plugins.googlesigninexample"
+        minSdkVersion 16
+        targetSdkVersion 28
+        versionCode flutterVersionCode.toInteger()
+        versionName flutterVersionName
+        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
+    }
+
+    buildTypes {
+        release {
+            // TODO: Add your own signing config for the release build.
+            // Signing with the debug keys for now, so `flutter run --release` works.
+            signingConfig signingConfigs.debug
+        }
+    }
+
+    testOptions {
+        unitTests.returnDefaultValues = true
+    }
+}
+
+flutter {
+    source '../..'
+}
+
+dependencies {
+    implementation 'com.google.android.gms:play-services-auth:16.0.1'
+    testImplementation'junit:junit:4.12'
+    testImplementation 'org.mockito:mockito-core:2.17.0'
+}