Value_Kind constructor Null safety

1. spine_client
2. struct.pb
3. Value_Kind
4. Value_Kind const constructor

Value_Kind enum

1. Constructors
2. Value_Kind
3. Values
4. nullValue
5. numberValue
6. stringValue
7. boolValue
8. structValue
9. listValue
10. notSet
11. Properties
12. hashCode
13. index
14. runtimeType
15. Methods
16. noSuchMethod
17. toString
18. Operators
19. operator ==
20. Constants
21. values