QueueEventType constructor Null safety

1. async_queue
2. async_queue
3. QueueEventType
4. QueueEventType const constructor

QueueEventType enum

1. Constructors
2. QueueEventType
3. Values
4. queueStart
5. beforeJob
6. afterJob
7. queueEnd
8. newJobAdded
9. queueClosed
10. queueStopped
11. violateAddWhenClosed
12. retryJob
13. retryLimitReached
14. Properties
15. hashCode
16. index
17. runtimeType
18. Methods
19. noSuchMethod
20. toString
21. Operators
22. operator ==
23. Constants
24. values