FirebaseClient constructor Null safety

1. spine_client
2. firebase_client
3. FirebaseClient
4. FirebaseClient constructor

FirebaseClient class

1. Constructors
2. FirebaseClient
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. childAdded
8. childChanged
9. childRemoved
10. get
11. noSuchMethod
12. toString
13. Operators
14. operator ==