This changes how SnackBar works so you can use it anywhere, not just on
the bottom edge of the screen (it used to rely on overflowing its bounds
and having negative offsets... I'm not really sure how hit testing
worked on it before!).

To do this I introduced a new RenderBox, RenderOverflowBox, that lets
you set your child's size independent of your own. I needed this so that
the snack bar could use a SquashTransition to change its size, while not
affecting the layout of its child. This is exposed as OverflowBox in
fn3. I'm not sure if it's the best API. It doesn't let you position the
child (which is an issue if the size you give is smaller), it doesn't
let you give a loose constraint (which maybe you might want?). But it
handles this use case, so for now it's probably ok.

Making the FAB get repositioned out of the way of the Snack Bar is now
done in the Scaffold, which is in charge of positioning both of those
and is the place that knows that both exist.

This patch makes a number of changes:

1) buildDirtyComponents now prevents all calls to setState, not just those
   higher in the tree. This change is necessary for consistency with
   MixedViewport and HomogeneousViewport because those widgets already build
   subwidgets with that restriction. If the "normal" build didn't enforce that
   rule, then some widgets would break when put inside a mixed or homogeneous
   viewport.

2) We now notify global key listeners in a microtask after beginFrame. That
   means setState is legal in these callbacks and that we'll produce another
   frame if someone calls setState in such a callback.

Update navigation example

Now this example uses the App widget to drive the adventure game.

Also:

- Make Dismissable report when it starts squashing, since otherwise we
  don't invalidate the list early enough and it gets mad that it wasn't
  told one of its children had changed size.

- Have Dismissable check that it gets removed after it's dismissed, to
  avoid having lots of redundant widgets around.

- Start tracking the height of each child of a MixedViewport, so that we
  don't accumulate floating point errors when a child jiggles up and down.

- Have _childOffsets reuse its storage space rather than newing up a new
  copy each time we reset the cache.

- Avoid double-updating child sizes when handling mixed viewport invalidations.

Add a currentWidget and currentState accessor to GlobalKeys

Add ensureWidgetIsVisible to fn3

Renames pointQuickDist in GameMath

Bumped Newton version to 0.1.4

Port some more tests to fn3

Add fling to Scrollable.

Port some more tests to fn3

Port Input and EditableText to fn3