ember data beta-12 compatibility

Unfortunately this change does break compatibility with ember data before beta 12. 
I could copy the [_notifyProperties function](https://github.com/emberjs/data/blob/d5f19518cb7b7e62b650ce6ce2329f9d0f1557e1/packages/ember-data/lib/system/model/model.js#L765) from ember data and that might restore compatibility.