itemChanged property Null safety

1. spine_client
2. subscription
3. StateSubscription<T extends GeneratedMessage>
4. itemChanged property

StateSubscription class

1. Constructors
2. of
3. Properties
4. closed
5. hashCode
6. itemAdded
7. itemChanged
8. itemRemoved
9. runtimeType
10. subscription
11. Methods
12. noSuchMethod
13. toString
14. unsubscribe
15. Operators
16. operator ==