postalCode property Null safety

1. baguette
2. Place
3. Place
4. postalCode property

Place class

1. Constructors
2. Place
3. Properties
4. address
5. city
6. country
7. hashCode
8. location
9. postalCode
10. runtimeType
11. Methods
12. noSuchMethod
13. toString
14. Operators
15. operator ==