R1:f36d504b7573

Use Array.prototype.slice in JX.$A

Summary: Being bold. I think it is fine to do this in 2011.

Test Plan: Run the new specs.

Reviewers: mroch, tomo, epriestley

Reviewed By: epriestley

Subscribers: chad, epriestley, aran

Differential Revision: https://secure.phabricator.com/D835