If you change the RenderObject tree between frames, you'll assert if
you subsequently hit test. So e.g. if you get two button presses back
to back, and you mutate the tree synchronously in response to the
first one, the second will assert.

This adds an onBuild callback to WidgetToRenderBoxAdapter to make it
easier to do the updates at the right time, i.e., during widget build.
It'll be called whenever you rebuild the WidgetToRenderBoxAdapter
itself, so all you have to do to use it is call setState() on whoever
is building the WidgetToRenderBoxAdapter.