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

Bug: v8:7220
Change-Id: I79022fe26c609225dce21c0b45e8bedff12cc7b5
Reviewed-on: https://chromium-review.googlesource.com/1076427Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53412}