clear method Null safety

1. Dart
2. dart:core
3. Set<E>
4. clear abstract method

Set class

1. Constructors
2. Set
3. from
4. identity
5. of
6. unmodifiable
7. Properties
8. first
9. hashCode
10. isEmpty
11. isNotEmpty
12. iterator
13. last
14. length
15. runtimeType
16. single
17. Methods
18. add
19. addAll
20. any
21. cast
22. clear
23. contains
24. containsAll
25. difference
26. elementAt
27. every
28. expand
29. firstWhere
30. fold
31. followedBy
32. forEach
33. intersection
34. join
35. lastWhere
36. lookup
37. map
38. noSuchMethod
39. reduce
40. remove
41. removeAll
42. removeWhere
43. retainAll
44. retainWhere
45. singleWhere
46. skip
47. skipWhile
48. take
49. takeWhile
50. toList
51. toSet
52. toString
53. union
54. where
55. whereType
56. Operators
57. operator ==
58. Static methods
59. castFrom