ENVIRONMENT_SANDBOX constant Null safety

1. baguette
2. CheckoutManager
3. CheckoutManager
4. ENVIRONMENT_SANDBOX constant

CheckoutManager class

1. Constructors
2. CheckoutManager
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 ==
16. Constants
17. BASE_URL_PRODUCTION
18. BASE_URL_SANDBOX
19. ENVIRONMENT_PRODUCTION
20. ENVIRONMENT_SANDBOX
21. STATUS_CODE_INVALID_DATA
22. STATUS_CODE_SUCCESS
23. STATUS_CODE_UNATHORIZED
24. STATUS_MESSAGE_INVALID_DATA
25. STATUS_MESSAGE_UNATHORIZED