It's extremely difficult to get right: there have been several bugs
related to this feature, especially when combined with
non-simple parameter lists in arrow functions.

BUG=chromium:727218

Change-Id: I97dfbc57a7650199964c5fe99de69143c8e537c2
Reviewed-on: https://chromium-review.googlesource.com/518145
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45603}