Enum constructor Null safety

1. Dart
2. dart:core
3. Enum
4. Enum constructor

Enum class

1. Constructors
2. Enum
3. Properties
4. hashCode
5. index
6. runtimeType
7. Methods
8. noSuchMethod
9. toString
10. Operators
11. operator ==
12. Static methods
13. compareByIndex
14. compareByName