fix: a bug with the demoProjectId arg to Firebase.initializeApp()

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

@@ -28,8 +28,15 @@ class Firebase {
     return _delegate.apps.map(FirebaseApp._).toList(growable: false);
   }
 
-  /// Initializes a new [FirebaseApp] instance by [name] and [options] and returns
-  /// the created app. This method should be called before any usage of FlutterFire plugins.
+  /// Initializes a new [FirebaseApp] instance by [name] and [options] and
+  /// returns the created app. This method should be called before any usage of
+  /// FlutterFire plugins.
+  ///
+  /// If a [demoProjectId] is provided, a new [FirebaseApp] instance will be
+  /// initialized with a set of default options for demo projects, overriding
+  /// the [options] argument. If no [name] is provided alongside a
+  /// [demoProjectId], the [demoProjectId] will be used as the app name. By
+  /// convention, the [demoProjectId] should begin with "demo-".
   ///
   /// The default app instance can be initialized here simply by passing no "name" as an argument
   /// in both Dart & manual initialization flows.
@@ -52,16 +59,21 @@ class Firebase {
         // We use 'web' as the default platform for unknown platforms.
         platformString = 'web';
       }
-      FirebaseAppPlatform app = await _delegate.initializeApp(
-        options: FirebaseOptions(
-          apiKey: '',
-          appId: '1:1:$platformString:1',
-          messagingSenderId: '',
-          projectId: demoProjectId,
-        ),
+      // A name must be set, otherwise [DEFAULT] will be used and the options
+      // we've provided will be ignored if any platform specific configuration
+      // files exist (i.e. GoogleService-Info.plist for iOS).
+      name ??= demoProjectId;
+      // The user should not set any options if they specify a demo project
+      // id, but it was allowed when this API was first added, so we allow it
+      // for backwards compatibility and simply override the user-provided
+      // options.
+      options = FirebaseOptions(
+        apiKey: '12345',
+        appId: '1:1:$platformString:1',
+        messagingSenderId: '',
+        projectId: demoProjectId,
       );
-
-      return FirebaseApp._(app);
+      // Now fall through to the normal initialization logic.
     }
     FirebaseAppPlatform app = await _delegate.initializeApp(
       name: name,

packages/firebase_core/firebase_core/test/firebase_core_test.dart

@@ -65,6 +65,45 @@ void main() {
       ]);
     });
   });
+
+  test('.initializeApp() with demoProjectId', () async {
+    const String demoProjectId = 'demo-project-id';
+    const String expectedName = demoProjectId;
+    const FirebaseOptions expectedOptions = FirebaseOptions(
+      apiKey: '12345',
+      // Flutter tests use android as the default platform.
+      appId: '1:1:android:1',
+      messagingSenderId: '',
+      projectId: demoProjectId,
+    );
+
+    final mock = MockFirebaseCore();
+    Firebase.delegatePackingProperty = mock;
+
+    final FirebaseAppPlatform platformApp =
+        FirebaseAppPlatform(expectedName, expectedOptions);
+
+    when(mock.apps).thenReturn([platformApp]);
+    when(mock.app(expectedName)).thenReturn(platformApp);
+    when(mock.initializeApp(name: expectedName, options: expectedOptions))
+        .thenAnswer((_) => Future.value(platformApp));
+
+    // Initialize the app with only a demo project id. The implementation will
+    // set the name and options accordingly.
+    FirebaseApp initializedApp = await Firebase.initializeApp(
+      demoProjectId: demoProjectId,
+    );
+    FirebaseApp app = Firebase.app(expectedName);
+
+    expect(initializedApp, app);
+    verifyInOrder([
+      mock.initializeApp(
+        name: expectedName,
+        options: expectedOptions,
+      ),
+      mock.app(expectedName),
+    ]);
+  });
 }
 
 class MockFirebaseCore extends Mock

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

@@ -121,9 +121,7 @@ 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 &&
-          !_options.projectId.startsWith('demo-')) {
+      if (defaultApp != null && _options != null) {
         if (_options.apiKey != defaultApp.options.apiKey ||
             (_options.databaseURL != null &&
                 _options.databaseURL != defaultApp.options.databaseURL) ||