fix(core): use valid-format placeholder options for demoProjectId apps

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

@@ -69,9 +69,9 @@ class Firebase {
       // for backwards compatibility and simply override the user-provided
       // options.
       resolvedOptions = FirebaseOptions(
-        apiKey: '12345',
+        apiKey: 'ADemoProjectApiKey000000000000000000000',
         appId: '1:1:$platformString:1',
-        messagingSenderId: '',
+        messagingSenderId: '1',
         projectId: demoProjectId,
       );
       // Now fall through to the normal initialization logic.

packages/firebase_core/firebase_core/test/firebase_core_test.dart

@@ -121,10 +121,10 @@ void main() {
     const String demoProjectId = 'demo-project-id';
     const String expectedName = demoProjectId;
     const FirebaseOptions expectedOptions = FirebaseOptions(
-      apiKey: '12345',
+      apiKey: 'ADemoProjectApiKey000000000000000000000',
       // Flutter tests use android as the default platform.
       appId: '1:1:android:1',
-      messagingSenderId: '',
+      messagingSenderId: '1',
       projectId: demoProjectId,
     );
 

tests/integration_test/firebase_core/firebase_core_e2e_test.dart

@@ -82,5 +82,14 @@ void main() {
         await app.setAutomaticResourceManagementEnabled(false);
       }
     });
+
+    test('Firebase.initializeApp() with demoProjectId', () async {
+      const demoProjectId = 'demo-flutterfire-e2e';
+      final app = await Firebase.initializeApp(demoProjectId: demoProjectId);
+      addTearDown(app.delete);
+
+      expect(app.name, demoProjectId);
+      expect(app.options.projectId, demoProjectId);
+    });
   });
 }