* globalToLocal was just broken when there was a rotation and a
  translation at the same time. This fixes that and adds a test.

* update graphic used by spinning_mixed since the old one went 404.

* simplify some of the code in the demo.

* fix MatrixUtils.transformPoint to be consistent with how we transform
  points elsewhere.

* stop transforming points elsewhere, just use
  MatrixUtils.transformPoint.

* make the Widget binding handle not having a root element.

* make the spinning_mixed demo update its widget tree.

Also, make sure that the parent is notified when they change.

Fixes #2298

There are still many more left before rendering.dart is complete.

It's safe to remove the unneeded `void`s from setters since the blocking issues in the
`always_declare_return_types` lint have been fixed (https://github.com/dart-lang/linter/).  We can also safely flip the bit on  `avoid_return_types_on_setters`.

I ran into a case where I was setting minHeight=∞ and then calling
layout() with that constraint, which is all kinds of bad. To try to
catch this earlier, this patch now provides a way to catch constraints
that are requiring infinite values.

We don't _always_ check this because there are valid uses for
BoxConstraints.biggest, e.g. as an additionalConstraint.

Transform applies its transform before painting, but sometimes you want
the widget to layout after its transform has been applied. We can't
handle general tranforms in this way because we can't couple width and
height constriants, but we can handle certain rotations.

Fixes #1214