holder property Null safety

1. baguette
2. BankCard
3. BankCard
4. holder property

BankCard class

1. Constructors
2. BankCard
3. Properties
4. cvv
5. expire
6. hashCode
7. holder
8. number
9. runtimeType
10. Methods
11. noSuchMethod
12. toString
13. Operators
14. operator ==