See the previous PR, interleaved mutation can cause values that were not
reactive to become reactive. I swear I had a case where this was observable, but
I came up with it before reordering the PRs in this stack. I think my repro
relied on an immutable reference to a mutable value, which is now handled in
InferReactivePlaces. So here i'm just adding fixtures, and allowing this case
since it's unobservable.