Allow sourcemap support even when XMLHttpRequest is being mocked

Summary:
While I was writing tests that use Sinon's `useFakeXMLHttpRequest`, I noticed
that source maps weren't being applied. This fixes that.

Test Plan:
In a test, use `sinon.useFakeXMLHttpRequest();` then throw an error. You should
still get sourcemaps applied.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D11139