Fix `next_src` being defined as a global variable

Summary: This variable should be local, not global. This silences a bunch of JSHint warnings.

Test Plan: `arc lint`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11067