[es6] ship @@isConcatSpreadable

[es6] ship @@isConcatSpreadable

As discussed, shipping early in the M48 branch will likely have greater
exposure than staging, in order to find any remaining bugs with this feature,
and provides plenty of time to unship if needed.

BUG=
LOG=N
R=adamk@chromium.org, wingo@chromium.org, rossberg@chromium.org

Committed: https://crrev.com/874549c79cd5f16bd997d0dfd5ee0301b052b0de
Cr-Commit-Position: refs/heads/master@{#31132}

[caitp (gmail), 2015-10-06 19:05:48]

See CL description for reasoning. I don't know if this is how we want to do it,
but maybe

[adamk, 2015-10-06 19:23:50]

lgtm

This seems fine to me.

[caitp (gmail), 2015-10-06 19:31:42]

The CQ bit was checked by caitpotter88@gmail.com

[commit-bot: I haz the power, 2015-10-06 19:31:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1391643003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1391643003/1

[commit-bot: I haz the power, 2015-10-06 19:56:39]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-06 19:57:01]

Patchset 1 (id:??) landed as
https://crrev.com/874549c79cd5f16bd997d0dfd5ee0301b052b0de
Cr-Commit-Position: refs/heads/master@{#31132}