Ember behaves differently on production variant.

If you're code relies on Em.keys() be aware that it will behave differently on the production variant unless you specify an ENV option.

Before you link ember, you need to set Ember = { ENV: { MANDATORY_SETTER: true }}; on a script tag.

<script src="//cdnjs.cloudflare.com/ajax/libs/handlebars.js/1.0.0/handlebars.min.js"></script>
<script>
  Ember = { ENV: { MANDATORY_SETTER: true }};
</script>
<script src="http://builds.emberjs.com/ember-latest.prod.js"></script>

Here is the issue with more context and detail.

https://github.com/emberjs/ember.js/issues/3691

I can't figure out why development and production variant would change that essential behavior. The answer might be deep into Ember.js internals. Who knows.