Rename all the "2" classes related to scrolling to remove the "2". Now that the
old scrolling code is gone, we don't need to use the suffix.

I plan to use this to implement similar logic in SliverPadding.

To make this easier to test I extended the paints matcher to accept a
function that takes a canvas. While I was at it I also made it accept
a Finder, it'll go and find the render object for you.

Also added support for paints..path and fixed some grammar in the
error messages.

Also improved the docs for debugPaint*.

These tests should hit some previously untested lines.