firstDate property Null safety

1. baguette
2. CalendarWidget
3. CalendarWidget
4. firstDate property

CalendarWidget class

1. Constructors
2. CalendarWidget
3. Properties
4. allowControls
5. contentAltColor
6. contentBorderColor
7. contentColor
8. contentTextColor
9. date
10. firstDate
11. hashCode
12. headerColor
13. headerTextColor
14. height
15. key
16. lastDate
17. margin
18. markerColor
19. markers
20. markerTextColor
21. onChangeDate
22. onLongPressMarker
23. onPressMarker
24. padding
25. runtimeType
26. width
27. Methods
28. createElement
29. createState
30. debugDescribeChildren
31. debugFillProperties
32. noSuchMethod
33. toDiagnosticsNode
34. toString
35. toStringDeep
36. toStringShallow
37. toStringShort
38. Operators
39. operator ==