Distilled repro of an internal example we found. Forget determines a mutable
range for the array, but that mutable range spans a hook call, so the reactive
scope gets pruned. That's all working as expected.
What isn't ideal though is that if we know `x` is an array and `f` can't mutate
its arguments, then `x.map(f)` shouldn't count as a mutation of `x`, since
Array.prototype.map can only mutate the receiver via the callback (if the
callback mutates its args).
Improving on this example requires a) we have to know it's an Array, via type
information or bc we saw an array literal and b) being precise about which
functions could possibly mutate their parameters, which is tricky because of
indirect mutations via stores, etc.