feat: add support for demo project (#11973)

* feat: add support for demo project

* format

* demo instructions

* improve docs

* doc

* fix iOS

* Update docs/setup/_setup_main.md

Co-authored-by: Kevin Cheung <kevinthecheung@users.noreply.github.com>

* fix ios

* format

* format

---------

Co-authored-by: Kevin Cheung <kevinthecheung@users.noreply.github.com>

Author: Lyokone

docs/setup/_setup_main.md

@@ -141,6 +141,16 @@ flutterfire configure
     flutter run
     ```
 
+If you would rather use a demo project, you can start the [Firebase Emulator](https://firebase.google.com/docs/emulator-suite) and
+in your `lib/main.dart` file initialize Firebase using `demoProjectId` (it should start with `demo-`):
+
+  ```dart
+  await Firebase.initializeApp(
+    demoProjectId: "demo-project-id",
+  );
+  ```
+
+
 
 ## **Step 4**: Add Firebase plugins {: #add-plugins}
 

packages/firebase_core/firebase_core/example/ios/Flutter/AppFrameworkInfo.plist

@@ -21,6 +21,6 @@
   <key>CFBundleVersion</key>
   <string>1.0</string>
   <key>MinimumOSVersion</key>
-  <string>11.0</string>
+  <string>12.0</string>
 </dict>
 </plist>

packages/firebase_core/firebase_core/example/ios/Runner.xcodeproj/project.pbxproj

@@ -166,7 +166,7 @@
 		97C146E61CF9000F007C117D /* Project object */ = {
 			isa = PBXProject;
 			attributes = {
-				LastUpgradeCheck = 1430;
+				LastUpgradeCheck = 1510;
 				ORGANIZATIONNAME = "The Chromium Authors";
 				TargetAttributes = {
 					97C146ED1CF9000F007C117D = {

packages/firebase_core/firebase_core/example/ios/Runner.xcodeproj/xcshareddata/xcschemes/Runner.xcscheme

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <Scheme
-   LastUpgradeVersion = "1430"
+   LastUpgradeVersion = "1510"
    version = "1.3">
    <BuildAction
       parallelizeBuildables = "YES"

packages/firebase_core/firebase_core/example/lib/firebase_options.dart

@@ -70,8 +70,6 @@ class DefaultFirebaseOptions {
     databaseURL:
         'https://flutterfire-e2e-tests-default-rtdb.europe-west1.firebasedatabase.app',
     storageBucket: 'flutterfire-e2e-tests.appspot.com',
-    androidClientId:
-        '406099696497-tvtvuiqogct1gs1s6lh114jeps7hpjm5.apps.googleusercontent.com',
     iosClientId:
         '406099696497-taeapvle10rf355ljcvq5dt134mkghmp.apps.googleusercontent.com',
     iosBundleId: 'io.flutter.plugins.firebase.tests',

packages/firebase_core/firebase_core/lib/firebase_core.dart

@@ -7,10 +7,10 @@ library firebase_core;
 
 import 'package:firebase_core_platform_interface/firebase_core_platform_interface.dart'
     hide MethodChannelFirebaseApp, MethodChannelFirebase;
-import 'package:flutter/widgets.dart';
+import 'package:flutter/foundation.dart';
 
 export 'package:firebase_core_platform_interface/firebase_core_platform_interface.dart'
     show FirebaseOptions, defaultFirebaseAppName, FirebaseException;
 
-part 'src/firebase_app.dart';
 part 'src/firebase.dart';
+part 'src/firebase_app.dart';

packages/firebase_core/firebase_core/lib/src/firebase.dart

@@ -39,7 +39,30 @@ class Firebase {
   static Future<FirebaseApp> initializeApp({
     String? name,
     FirebaseOptions? options,
+    String? demoProjectId,
   }) async {
+    if (demoProjectId != null) {
+      late final String platformString;
+      if (defaultTargetPlatform == TargetPlatform.android) {
+        platformString = 'android';
+      } else if (defaultTargetPlatform == TargetPlatform.iOS ||
+          defaultTargetPlatform == TargetPlatform.macOS) {
+        platformString = 'ios';
+      } else {
+        // We use 'web' as the default platform for unknown platforms.
+        platformString = 'web';
+      }
+      FirebaseAppPlatform app = await _delegate.initializeApp(
+        options: FirebaseOptions(
+          apiKey: '',
+          appId: '1:1:$platformString:1',
+          messagingSenderId: '',
+          projectId: demoProjectId,
+        ),
+      );
+
+      return FirebaseApp._(app);
+    }
     FirebaseAppPlatform app = await _delegate.initializeApp(
       name: name,
       options: options,

packages/firebase_core/firebase_core_platform_interface/lib/src/method_channel/method_channel_firebase.dart

@@ -124,7 +124,9 @@ class MethodChannelFirebase extends FirebasePlatform {
       // check to see if options are roughly identical (so we don't unnecessarily
       // throw on minor differences such as platform specific keys missing
       // e.g. hot reloads/restarts).
-      if (defaultApp != null && _options != null) {
+      if (defaultApp != null &&
+          _options != null &&
+          !_options.projectId.startsWith('demo-')) {
         if (_options.apiKey != defaultApp.options.apiKey ||
             (_options.databaseURL != null &&
                 _options.databaseURL != defaultApp.options.databaseURL) ||