If an initializer is a NaryOperation, its position ends up as a start position
of a Scope, and a DCHECK used to fire.

Interestingly, this was not caught by our existing tests.

BUG=chromium:791256

Change-Id: Id47f850c7ad17ca580352f9bd56c9567b485c3b8
Reviewed-on: https://chromium-review.googlesource.com/822093Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50051}