This way, we can also check the return code of d8. We currently have a
bug (6981) which makes failing tests not being detected, even though
the failure message is (sometimes) being printed.
After this refactoring, we can write tests for our mjsunit test
functions.

R=machenbach@chromium.org

Bug: v8:6981
Change-Id: I0aa0abcb0f9a4f622a1e1d1a4d826da1e6eb4f07
Reviewed-on: https://chromium-review.googlesource.com/737991Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48951}

Only allow BindingIdentifier in BindingRestPattern and 
ValidReferenceExpression in AssignmentRestPattern.

Also updated to a better, actionable error message.

Bug: v8:6500, v8:6513
Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: Ifaba2f85c20bc20e263267e8c76d50a27075b87d
Reviewed-on: https://chromium-review.googlesource.com/550559
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46653}