Add missing `this`

`store` is undefined in this context, making the assertion blow up.

Reproduction case:

```js
var User = DS.ModelFragment.extend({name: DS.attr('string')});
var Comment = DS.Model.extend({
  user: DS.hasOneFragment('user')
});

// later, somewhere with a store:
var user = this.store.createFragment('user', {name: 'test'});

// this errors out with `store is undefined`
var comment = this.store.createRecord('comment', {user: user});
```