ConstDurations constructor Null safety

1. mc_gallery
2. const_durations
3. ConstDurations
4. ConstDurations constructor

ConstDurations class

1. Constructors
2. ConstDurations
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toString
9. Operators
10. operator ==
11. Constants
12. defaultAnimationDuration
13. doubleDefaultAnimationDuration
14. halfDefaultAnimationDuration
15. oneAndHalfDefaultAnimationDuration
16. quarterDefaultAnimationDuration
17. tripleDefaultAnimationDuration
18. zero