This makes the sync code stop if it would have to rearrange the
RenderObjects. I'll make it handle the cross-RenderObject case, as well
as the insertion-sync case, in subsequent patches.
This should handle a case like a stateful component inside a Container
inside another Container having one of those Containers removed while
still keeping that stateful component around with its state.
The problem of how to handle the Container then being reinserted is a
separate issue not handled by this patch.
