requestToken abstract method Null safety

1. baguette
2. ICheckoutManager
3. ICheckoutManager
4. requestToken abstract method

ICheckoutManager class

1. Constructors
2. ICheckoutManager
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. addPlugin
8. cleanPlugins
9. getPlugin
10. noSuchMethod
11. removePlugin
12. requestToken
13. toString
14. Operators
15. operator ==