Example app is broken.

### Error
```
lib/main.dart:9:8: Error: Error when reading 'lib/firebase_options.dart': No such file or directory
import 'firebase_options.dart';
       ^
lib/main.dart:14:14: Error: Undefined name 'DefaultFirebaseOptions'.
    options: DefaultFirebaseOptions.currentPlatform,
             ^^^^^^^^^^^^^^^^^^^^^^
```