UnmodifiableSetMixin<E> constructor Null safety

1. Flutter
2. collection
3. UnmodifiableSetMixin<E>
4. UnmodifiableSetMixin constructor

UnmodifiableSetMixin class

1. Constructors
2. UnmodifiableSetMixin
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. addAll
16. any
17. cast
18. clear
19. contains
20. containsAll
21. difference
22. elementAt
23. every
24. expand
25. firstWhere
26. fold
27. followedBy
28. forEach
29. intersection
30. join
31. lastWhere
32. lookup
33. map
34. noSuchMethod
35. reduce
36. remove
37. removeAll
38. removeWhere
39. retainAll
40. retainWhere
41. singleWhere
42. skip
43. skipWhile
44. take
45. takeWhile
46. toList
47. toSet
48. toString
49. union
50. where
51. whereType
52. Operators
53. operator ==