Previously, if a StatefulWidget was marked dirty, then removed from the
build, then reinserted using the exact same widget under a widget under
a LayoutBuilder, it wouldn't rebuild.
This fixes that.
It also introduces an assert that's supposed to catch SizeObserver-like
behaviour. Rather than make this patch even bigger, I papered over two
pre-existing bugs which this assert uncovered (and fixed the other
problems it found):
https://github.com/flutter/flutter/issues/5751https://github.com/flutter/flutter/issues/5749
We should fix those before 1.0 though.
