Having UniqueComponent automatically generate its own key is a trap. If
anyone ever creates a UniqueComponent in a build function (rather than
ahead of time) and forgets to pass a key, then that entire subtree is
going to be rebuilt, including layout, every time it's updated. Since
there's basically no way for us to catch this, we should at least force
the author to see the explicit "new GlobalKey()" call in their code.

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