While working in https://pub.dev/packages/two_dimensional_scrollables, I found I could eliminate some casts if we added covariants here in the framework.
Much of the 2D aPI in the framework is abstract, so I think it make sense to add these and make it easier/cleaner for subclasses using the APIs.
I made a similar change in https://github.com/flutter/flutter/pull/131358, this would cover all of the cases I could find so its nice and accommodating now.