MODEL_FACTORY_INJECTIONS needs to be consulted for polymorphic type assertions

when building via *ember-cli*, the factory that is checked in these spots is different because of how the resolver works.

Original work by @krisselden and I updated it to post SSOT merge.  cc: @lukemelia 
Can we use `modelFor` here instead of the container directly? There is a serious leaking of problem space if we reference `Ember.MODEL_FACTORY_INJECTIONS` directly in Ember Data