BackButtonDispatcher constructor Null safety

1. Flutter
2. widgets
3. BackButtonDispatcher
4. BackButtonDispatcher constructor

BackButtonDispatcher class

1. Constructors
2. BackButtonDispatcher
3. Properties
4. hasCallbacks
5. hashCode
6. runtimeType
7. Methods
8. addCallback
9. createChildBackButtonDispatcher
10. deferTo
11. forget
12. invokeCallback
13. noSuchMethod
14. removeCallback
15. takePriority
16. toString
17. Operators
18. operator ==