Tapping on the menu item didn't animate the checkbox because the menu
takes 300ms to animate entirely away while the checkbox takes 200ms to
animate checked, and since the item with the checkbox was at the bottom,
we were only seeing about 60ms of the entire checkbox animation, which
isn't enough to notice it. So I moved it to the top of the menu.

Tapping on the checkbox didn't animate because nothing caused the menu
to rebuild when the callback was invoked. To trigger a rebuild, I now
call navigator.setState() explicitly, after changing out local state.

To make tapping the checkbox remove the menu, I also explicitly call
navigator.pop() in the code that handles the check. (I still explicitly
change the checkbox to show that that's possible. In principle one could
just treat the checkbox as an inert widget that happens to trigger pop,
and do all the checking/unchecking in the switch statement.)

I also made some minor style tweaks to files I was looking at while
dealing with this issue.

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