The TC39 committee reached consensus to rename `flatten` to `flat`
during the May 22nd meeting. The corresponding patch to the proposal
is here:

https://github.com/tc39/proposal-flatMap/commit/093eacc7fe0906e70f7626bf6c7d6e9dfc53cce9

Bug: v8:7220
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: Ie8049ae4d4589a4ae7fe3d203053cef798c135e4
Reviewed-on: https://chromium-review.googlesource.com/1069467Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53294}