ScrollMetrics class

1. Constructors
2. ScrollMetrics
3. Properties
4. atEdge
5. axis
6. axisDirection
7. extentAfter
8. extentBefore
9. extentInside
10. maxScrollExtent
11. minScrollExtent
12. outOfRange
13. pixels
14. viewportDimension
15. hashCode
16. runtimeType
17. Methods
18. copyWith
19. noSuchMethod
20. toString
21. Operators
22. operator ==