HistoryBase constructor Null safety

1. Flutter
2. dart:html
3. HistoryBase
4. HistoryBase constructor

HistoryBase class

1. Constructors
2. HistoryBase
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. back
8. forward
9. go
10. noSuchMethod
11. toString
12. Operators
13. operator ==