Before 983eec89, RewritableExpressions
which had been queued for destructuring assignment rewriting but which
turned out to be part of a binding pattern in arrow function parameters
would be silently ignored by the PatternRewriter. After that CL, they
failed with a DCHECK.

This patch reverts to the previous behavior, with a TODO to handle this
in a better way by dequeuing RewritableExpressions that turned out
to be part of an inner arrow function.

Bug: chromium:756332
Change-Id: I0a9bf51499940c944034d9a8128e89950de38059
Reviewed-on: https://chromium-review.googlesource.com/619506Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47435}