Ember strict mode

Hey so I am developing an Ember application and am seeing seemingly random strict mode behavior. What I mean is that as I am developing, other than jshint errors, I don't see any effects of when I break a strict mode rule, ie duplicate properties on an object. Even in prod builds I don't see it in Chrome, but a tester sees it in IE and I quickly make the fix. A different developer though sees this issue as she is working with the built in Ember-cli server.

So how do I make sure that I am running in strict mode?