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 
Shouldn't `belongsToType` be the container factory instead of the base class? That seems like the correct fix, otherwise we'll be whacking moles across the codebase.