Using Isolate is unsafe, because we might parse (and do scope analysis)
on a background thread.

The illegal access happens when encountering f(arguments) { ... }.

Kudos to verwaest@ for finding this bug.

R=verwaest@chromium.org, rossberg@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2158343002
Cr-Commit-Position: refs/heads/master@{#37893}