Flag is used by followup patches split apart from
https://codereview.chromium.org/2622833002/, and tests for each split
out CL ends up using Symbol.asyncIterator, so it makes sense to land it
first (behind a flag).

BUG=v8:5855
R=littledan@chromium.org, adamk@chromium.org

Review-Url: https://codereview.chromium.org/2645923003
Cr-Commit-Position: refs/heads/master@{#42527}