DefaultFirebaseOptions constructor Null safety

1. firebase_ui_example
2. firebase_options
3. DefaultFirebaseOptions
4. DefaultFirebaseOptions constructor

DefaultFirebaseOptions class

1. Constructors
2. DefaultFirebaseOptions
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toString
9. Operators
10. operator ==
11. Static properties
12. currentPlatform
13. Constants
14. android
15. ios
16. macos
17. web