The spec requires that we use IterableToList, which we skipped for
some arrays as an optimization. We can't skip this for arrays with
objects though, because the objects may mutate the array during
the copying step via valueOf side effects.

Also clean up the implementation to use a runtime function rather
than a builtin as the helper. Also reverses the result of the helper
because I think it is a bit more intuitive that way.

Bug: v8:6224
Change-Id: I9199491abede4479785df6d9068331bc2d6e9c5e
Reviewed-on: https://chromium-review.googlesource.com/471986Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44507}