HashedObserverList<T> constructor Null safety

1. Flutter
2. foundation
3. HashedObserverList<T>
4. HashedObserverList constructor

HashedObserverList class

1. Constructors
2. HashedObserverList
3. Properties
4. first
5. hashCode
6. isEmpty
7. isNotEmpty
8. iterator
9. last
10. length
11. runtimeType
12. single
13. Methods
14. add
15. any
16. cast
17. contains
18. elementAt
19. every
20. expand
21. firstWhere
22. fold
23. followedBy
24. forEach
25. join
26. lastWhere
27. map
28. noSuchMethod
29. reduce
30. remove
31. singleWhere
32. skip
33. skipWhile
34. take
35. takeWhile
36. toList
37. toSet
38. toString
39. where
40. whereType
41. Operators
42. operator ==