CssClassSet constructor Null safety

1. Dart
2. dart:html
3. CssClassSet
4. CssClassSet constructor

CssClassSet class

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