These properties had a high cost in the documentation because they're
all top-level properties, and there's really very little reason for
these values to be configurable in the first place.
It was 8.0. It's now arbitrarily 18.0.
Changing this required adjusting some tests. Adjusting the tests
required debugging the tests. Debugging the tests required some tools
to help debugging gesture recognizers and gesture arenas, so I added
some. It also required updating some toString() methods which resulted
in some changes to the tree diagnostics logic.
Also I cleaned up some docs while I was at it.
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*.
