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.