ValidityState class

1. Properties
2. badInput
3. customError
4. patternMismatch
5. rangeOverflow
6. rangeUnderflow
7. stepMismatch
8. tooLong
9. tooShort
10. typeMismatch
11. valid
12. valueMissing
13. hashCode
14. runtimeType
15. Methods
16. noSuchMethod
17. toString
18. Operators
19. operator ==