Automattic/jetpack — pull request #1094 by jblz (2014-09-05 20:03:43+00:00)
Title: Infinite Scroll: early set the DOING_AJAX constant.

Do it when the plugin is sourced -- don't wait until the `template_redirect` action.
Lets plugins / themes rely on the constant in prior actions (like `init`).
See issue: #1093