My RenderSliverMultiBoxAdaptor/RenderSliverFixedExtentList/RenderSliverVariedExtentList yak shave continues.

I've been subclassing RenderSliverVariedExtentList for SliverTree and have found some opportunities for clean up.
There is a larger clean up I'd like to do, but this week SliverTree comes first. 

I noticed these methods were getting repeated 🔁, and I was about to repeat them again 🔁 for the tree, so I figured bumping them up to the base class was better than continuing to copy-paste the same methods.