ObserverList<T> constructor Null safety

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

ObserverList class

1. Constructors
2. ObserverList
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. clear
18. contains
19. elementAt
20. every
21. expand
22. firstWhere
23. fold
24. followedBy
25. forEach
26. join
27. lastWhere
28. map
29. noSuchMethod
30. reduce
31. remove
32. singleWhere
33. skip
34. skipWhile
35. take
36. takeWhile
37. toList
38. toSet
39. toString
40. where
41. whereType
42. Operators
43. operator ==