Proposal repo: https://tc39.github.io/proposal-flatMap/

Bug: v8:7220
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: I61661fc6d5c39d084ce5c96a9e150e5c26799e2d
Also-By: bmeurer@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/957043
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51967}