Without this the call to `formatFailureText` in `test-async.js`
fails but goes unnoticed since the promise change is rejects
which is not handled.  And d8 silently ignores the the unhandled
rejections.

Once `formatFailureText` was added it reveals a but where several
tests were expecting `.equal` to be a deepEquals.  Specifically:

test/mjsunit/es6/promise-all.js
test/mjsunit/harmony/async-generators-resume-return.js
test/mjsunit/harmony/async-generators-return.js
test/mjsunit/harmony/async-generators-yield.js

Making equals call `deepEquals` fixed that issue.

Change-Id: I350c7d916147eaa7cf873bdaf273aebbaaa833c5
Reviewed-on: https://chromium-review.googlesource.com/1236852
Commit-Queue: Sam Clegg <sbc@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56107}