Migration constructor Null safety

1. conduit_core
2. conduit
3. Migration
4. Migration constructor

Migration class

1. Constructors
2. Migration
3. Properties
4. currentSchema
5. database
6. hashCode
7. runtimeType
8. store
9. version
10. Methods
11. downgrade
12. noSuchMethod
13. seed
14. toString
15. upgrade
16. Operators
17. operator ==
18. Static methods
19. sourceForSchemaUpgrade