size property Null safety

1. nativebase_flutter
2. heading
3. Heading
4. size property

Heading class

1. Constructors
2. Heading
3. Properties
4. color
5. fontFamily
6. fontSize
7. fontStyle
8. fontWeight
9. hashCode
10. key
11. letterSpacing
12. lineHeight
13. runtimeType
14. size
15. text
16. textAlign
17. textDecoration
18. textTransform
19. Methods
20. build
21. createElement
22. debugDescribeChildren
23. debugFillProperties
24. noSuchMethod
25. resolveComponentLevelTheme
26. resolveProps
27. resolveTokens
28. toDiagnosticsNode
29. toJson
30. toString
31. toStringDeep
32. toStringShallow
33. toStringShort
34. Operators
35. operator ==