Assertiveness constructor Null safety

1. Flutter
2. semantics
3. Assertiveness
4. Assertiveness const constructor

Assertiveness enum

1. Constructors
2. Assertiveness
3. Values
4. polite
5. assertive
6. Properties
7. hashCode
8. index
9. runtimeType
10. Methods
11. noSuchMethod
12. toString
13. Operators
14. operator ==
15. Constants
16. values