StreamSinkBase<T> constructor Null safety

1. Flutter
2. async
3. StreamSinkBase<T>
4. StreamSinkBase constructor

StreamSinkBase class

1. Constructors
2. StreamSinkBase
3. Properties
4. done
5. hashCode
6. runtimeType
7. Methods
8. add
9. addError
10. addStream
11. close
12. noSuchMethod
13. onAdd
14. onClose
15. onError
16. toString
17. Operators
18. operator ==