[BUGFIX release] Manual merge PR for #4717 into stable branch

@rjackson per our convo on IRC, here's https://github.com/emberjs/ember.js/pull/4717 in a form that can be merged into the [stable branch](https://github.com/emberjs/ember.js/tree/stable).

**This currently will fail builds on a *real* browser because of https://github.com/emberjs/ember.js/pull/4730**

![Manual merging isn't fair image](http://i.imgur.com/yNKR9ou.gif)