PREVIOUS_SCREEN constant Null safety

1. baguette
2. ScreenKey
3. ScreenKey
4. PREVIOUS_SCREEN constant

ScreenKey class

1. Constructors
2. ScreenKey
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toString
9. Operators
10. operator ==
11. Constants
12. ENVIRONMENT_SCREEN
13. PREVIOUS_SCREEN
14. ROOT_SCREEN
15. TAG