expandBackAll method Null safety

1. Flutter
2. characters
3. CharacterRange
4. expandBackAll abstract method

CharacterRange class

1. Constructors
2. CharacterRange
3. at
4. Properties
5. charactersAfter
6. charactersBefore
7. current
8. currentCharacters
9. hashCode
10. isEmpty
11. isNotEmpty
12. runes
13. runtimeType
14. source
15. stringAfter
16. stringAfterLength
17. stringBefore
18. stringBeforeLength
19. utf16CodeUnits
20. Methods
21. collapseToEnd
22. collapseToFirst
23. collapseToLast
24. collapseToStart
25. copy
26. dropBackTo
27. dropBackUntil
28. dropBackWhile
29. dropFirst
30. dropLast
31. dropTo
32. dropUntil
33. dropWhile
34. endsWith
35. expandAll
36. expandBack
37. expandBackAll
38. expandBackTo
39. expandBackUntil
40. expandBackWhile
41. expandNext
42. expandTo
43. expandUntil
44. expandWhile
45. isFollowedBy
46. isPrecededBy
47. moveBack
48. moveBackAll
49. moveBackTo
50. moveBackUntil
51. moveNext
52. moveNextAll
53. moveTo
54. moveUntil
55. noSuchMethod
56. replaceAll
57. replaceFirst
58. replaceRange
59. split
60. startsWith
61. toString
62. Operators
63. operator ==