https://github.com/tc39/proposal-flatMap/issues/74
https://github.com/tc39/proposal-flatMap/pull/75

BUG=v8:7220

Change-Id: I93ce66bddbb429beaa7486311f120ec32d6a35fb
Reviewed-on: https://chromium-review.googlesource.com/c/1390125Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58537}