dark property Null safety

1. nativebase_flutter
2. color_model
3. NativeBaseColor
4. dark property

NativeBaseColor class

1. Constructors
2. NativeBaseColor
3. Properties
4. amber
5. black
6. blue
7. blueGray
8. coolGray
9. cyan
10. dark
11. darkBlue
12. emerald
13. fuchsia
14. gray
15. green
16. hashCode
17. indigo
18. lightBlue
19. lime
20. orange
21. pink
22. primary
23. purple
24. red
25. rose
26. runtimeType
27. teal
28. text
29. trueGray
30. warmGray
31. white
32. yellow
33. Methods
34. noSuchMethod
35. toJson
36. toString
37. Operators
38. operator ==