DownloadStatus constructor Null safety

1. agora_chat_sdk
2. agora_chat_sdk
3. DownloadStatus
4. DownloadStatus const constructor

DownloadStatus enum

1. Constructors
2. DownloadStatus
3. Values
4. PENDING
5. DOWNLOADING
6. SUCCESS
7. FAILED
8. Properties
9. hashCode
10. index
11. runtimeType
12. Methods
13. noSuchMethod
14. toString
15. Operators
16. operator ==
17. Constants
18. values