AreaCode constructor Null safety

1. agora_chat_sdk
2. agora_chat_sdk
3. AreaCode
4. AreaCode constructor

AreaCode class

1. Constructors
2. AreaCode
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toString
9. Operators
10. operator ==
11. Constants
12. AS
13. CN
14. EU
15. GLOB
16. IN
17. JP
18. NA