lowerSurface property Null safety

1. jrpn
2. view.buttons
3. CalculatorButton
4. lowerSurface property

CalculatorButton class

1. Constructors
2. CalculatorButton
3. Properties
4. acceleratorKey
5. acceleratorLabel
6. bFactory
7. fKey
8. fText
9. gKey
10. gText
11. gTextOffset
12. hashCode
13. height
14. innerBorder
15. innerBorderColor
16. key
17. keyTextOffset
18. keyTextStyle
19. lowerSurface
20. lowerSurfaceColor
21. lowerSurfaceColorPressed
22. outerBorder
23. outerBorderPressedScale
24. runtimeType
25. uKey
26. upperSurface
27. upperSurfaceColor
28. upperSurfaceColorPressed
29. uText
30. width
31. Methods
32. createElement
33. createState
34. debugDescribeChildren
35. debugFillProperties
36. drawBlueText
37. drawGoldText
38. drawKeyboardAccelerator
39. drawText
40. drawWhiteText
41. noSuchMethod
42. paintForPainter
43. toDiagnosticsNode
44. toString
45. toStringDeep
46. toStringShallow
47. toStringShort
48. Operators
49. operator ==