docs(firebase_app_check): Add debug provider documentation.

docs/app-check/debug-provider.md

@@ -1,7 +1,9 @@
-Project: /docs/_project.yaml
+Project: /docs/app-check/_project.yaml
 Book: /docs/_book.yaml
+page_type: guide
 
-<link rel="stylesheet" type="text/css" href="/styles/docs.css" />
+{% include "docs/app-check/_local_variables.html" %}
+{% include "_shared/firebase/_snippet_include_comment.html" %}
 
 #  Use App Check with the debug provider with Flutter
 
@@ -16,9 +18,165 @@ Warning: The debug provider allows access to your Firebase resources from
 unverified devices. Don't use the debug provider in production builds of your
 app, and don't share your debug builds with untrusted parties.
 
-The debug provider does not currently have a Dart API; you'll need to apply the
-changes individually for each of your platforms:
+## Apple platforms
 
-- [Use App Check with the debug provider on Apple platforms](/docs/app-check/ios/debug-provider)
-- [Use App Check with the debug provider on Android](/docs/app-check/android/debug-provider)
-- [Use App Check with the debug provider in web apps](/docs/app-check/web/debug-provider)
+To use the debug provider while running your app in a simulator interactively
+(during development, for example), do the following:
+
+1.  In the file `ios/Runner/AppDelegate.swift` (or `AppDelegate.m`), create and
+    set the App Check debug provider factory in Debug builds:
+
+    * {Swift}
+
+      ```swift
+      import UIKit
+      import Flutter
+      import Firebase  // Add the Firebase import.
+
+      @UIApplicationMain
+      @objc class AppDelegate: FlutterAppDelegate {
+        override func application(
+          _ application: UIApplication,
+          didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
+        ) -> Bool {
+          // Use the debug provider in Debug builds:
+      #if DEBUG
+          let providerFactory = AppCheckDebugProviderFactory()
+          AppCheck.setAppCheckProviderFactory(providerFactory)
+      #endif
+
+          GeneratedPluginRegistrant.register(with: self)
+          return super.application(application, didFinishLaunchingWithOptions: launchOptions)
+        }
+      }
+      ```
+
+      Tip: Confirm in your Xcode project's Build Settings that it passes the
+      `DEBUG` flag to the Swift compiler.
+
+    * {Objective-C}
+
+      ```objc
+      #import "AppDelegate.h"
+      #import "GeneratedPluginRegistrant.h"
+      @import Firebase;  // Add the Firebase import.
+
+      @implementation AppDelegate
+
+      - (BOOL)application:(UIApplication *)application
+          didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
+        // Use the debug provider in Debug builds:
+      #if DEBUG
+        FIRAppCheckDebugProviderFactory *providerFactory = [[FIRAppCheckDebugProviderFactory alloc] init];
+        [FIRAppCheck setAppCheckProviderFactory:providerFactory];
+      #endif
+        [GeneratedPluginRegistrant registerWithRegistry:self];
+        // Override point for customization after application launch.
+        return [super application:application didFinishLaunchingWithOptions:launchOptions];
+      }
+
+      @end
+      ```
+
+1.  Enable debug logging in your Xcode project (v11.0 or newer):
+
+    1.  Open **Product > Scheme > Edit scheme**.
+    1.  Select **Run** from the left menu, then select the **Arguments** tab.
+    1.  In the **Arguments Passed on Launch** section, add `-FIRDebugEnabled`.
+
+1.  Open `ios/Runner.xcworkspace` with Xcode and run your app in the Simulator.
+    Your app will print a local debug token to the debug output when Firebase
+    tries to send a request to the backend. For example:
+
+    <pre>[Firebase/AppCheck][I-FAA001001] Firebase App Check Debug Token:
+    123a4567-b89c-12d3-e456-789012345678</pre>
+
+{# Google-internal common file: #}
+<<../_includes/manage-debug-tokens.md>>
+
+After you register the token, Firebase backend services will accept it as valid.
+
+Because this token allows access to your Firebase resources without a
+valid device, it is crucial that you keep it private. Don't commit it to a
+public repository, and if a registered token is ever compromised, revoke it
+immediately in the Firebase console.
+
+## Android
+
+To use the debug provider while running your Flutter app in an Android environment, 
+implement the following code in your Flutter application:
+
+```dart
+import 'package:flutter/material.dart';
+import 'package:firebase_core/firebase_core.dart';
+
+// Import the firebase_app_check plugin
+import 'package:firebase_app_check/firebase_app_check.dart';
+
+Future<void> main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  await Firebase.initializeApp();
+  await FirebaseAppCheck.instance.activate(
+    webRecaptchaSiteKey: 'recaptcha-v3-site-key',
+    // Set the androidDebugProvider to `true`
+    androidDebugProvider: true,
+  );
+  runApp(App());
+}
+
+```
+
+Your app will print a local debug token to the debug output when Firebase tries
+to send a request to the backend. For example:
+
+<pre>D DebugAppCheckProvider: Enter this debug secret into the allow list in
+the Firebase Console for your project: 123a4567-b89c-12d3-e456-789012345678</pre>
+
+{# Google-internal common file: #}
+<<../_includes/manage-debug-tokens.md>>
+
+After you register the token, Firebase backend services will accept it as valid.
+
+## Web
+
+To use the debug provider while running your app from `localhost` (during
+development, for example), do the following:
+
+Warning: _Do not_ try to enable `localhost` debugging by adding `localhost` to
+reCAPTCHA’s allowed domains. Doing so would allow anyone to run your app from
+their local machines!
+
+1.  In the file `web/index.html`, enable debug mode by setting
+    `self.FIREBASE_APPCHECK_DEBUG_TOKEN` to `true`:
+
+    ```html
+    <body>
+      <script>
+        self.FIREBASE_APPCHECK_DEBUG_TOKEN = true;
+      </script>
+
+      ...
+
+    </body>
+    ```
+
+1.  Run your web app locally and open the browser’s developer tool. In the
+    debug console, you’ll see a debug token:
+
+    <pre>AppCheck debug token: "123a4567-b89c-12d3-e456-789012345678". You will
+    need to safelist it in the Firebase console for it to work.</pre>
+
+    This token is stored locally in your browser and will be used whenever you
+    use your app in the same browser on the same machine. If you want to use the
+    token in another browser or on another machine, set
+    `self.FIREBASE_APPCHECK_DEBUG_TOKEN` to the token string instead of `true`.
+
+{# Google-internal common file: #}
+<<../_includes/manage-debug-tokens.md>>
+
+After you register the token, Firebase backend services will accept it as valid.
+
+Because this token allows access to your Firebase resources without a
+valid device, it is crucial that you keep it private. Don't commit it to a
+public repository, and if a registered token is ever compromised, revoke it
+immediately in the Firebase console.