flipCollapseButtonIcon property Null safety

1. markdown_toolbar
2. markdown_toolbar
3. MarkdownToolbar
4. flipCollapseButtonIcon property

MarkdownToolbar class

1. Constructors
2. MarkdownToolbar
3. Properties
4. alignCollapseButtonEnd
5. alignment
6. backgroundColor
7. boldCharacter
8. boldTooltip
9. bulletedListCharacter
10. bulletedListTooltip
11. checkboxTooltip
12. codeCharacter
13. codeTooltip
14. collapsable
15. controller
16. dropdownTextColor
17. flipCollapseButtonIcon
18. focusNode
19. hashCode
20. headingTooltip
21. height
22. hideBold
23. hideBulletedList
24. hideCheckbox
25. hideCode
26. hideHeading
27. hideHorizontalRule
28. hideImage
29. hideItalic
30. hideLink
31. hideNumberedList
32. hideQuote
33. hideStrikethrough
34. horizontalRuleCharacter
35. horizontalRuleTooltip
36. iconColor
37. iconSize
38. imageTooltip
39. italicCharacter
40. italicTooltip
41. key
42. linkTooltip
43. numberedListTooltip
44. quoteTooltip
45. runSpacing
46. runtimeType
47. showTooltips
48. spacing
49. strikethroughTooltip
50. useIncludedTextField
51. width
52. Methods
53. createElement
54. createState
55. debugDescribeChildren
56. debugFillProperties
57. noSuchMethod
58. toDiagnosticsNode
59. toString
60. toStringDeep
61. toStringShallow
62. toStringShort
63. Operators
64. operator ==