WritePipe constructor Null safety

1. Dart
2. dart:io
3. WritePipe
4. WritePipe constructor

WritePipe class

1. Constructors
2. WritePipe
3. Properties
4. done
5. encoding
6. hashCode
7. runtimeType
8. Methods
9. add
10. addError
11. addStream
12. close
13. flush
14. noSuchMethod
15. toString
16. write
17. writeAll
18. writeCharCode
19. writeln
20. Operators
21. operator ==