key property Null safety

1. jrpn
2. model
3. KeyArg
4. key property

KeyArg class

1. Constructors
2. KeyArg
3. Properties
4. child
5. hashCode
6. key
7. runtimeType
8. Methods
9. init
10. matches
11. noSuchMethod
12. toString
13. Operators
14. operator ==