toJson method Null safety

1. baguette
2. Currency
3. Currency
4. toJson method

Currency class

1. Constructors
2. Currency
3. fromJson
4. Properties
5. code
6. hashCode
7. name
8. runtimeType
9. symbol
10. Methods
11. noSuchMethod
12. toJson
13. toString
14. Operators
15. operator ==