[Fail at async, please advise ] Add assertion test

Hi:

Add an assertion test for Ember.assert when model is not defined for route with dynamic segment.

Although this test passes, it pollutes other async integration tests in `basic-test`:

> Uncaught TypeError: Cannot call method 'find' of undefined
Source: 	
http://localhost:9292/?package=ember:352

I believe the cause of the error are:
* Even though `Ember.assert` is triggered, the flow continues to `return modelClass.find(value)` with modelClass is nil. 
* store computed property didn't return right away. 

Please advise how to tackle the test pollution? Much appreciated!

Below is what I'm trying to test:
`````
 store: Ember.computed(function(){
    var container = this.container;
    var routeName = this.routeName;
    var namespace = get(this, 'router.namespace');

    return {
      find: function(name, value) {
        var modelClass = container.lookupFactory('model:' + name);

        Ember.assert("You used the dynamic segment " + name + "_id in your route "+ routeName + ", but " + namespace + "." + classify(name) + " did not exist and you did not override your route's `model` hook.", modelClass);

        return modelClass.find(value);
      }
    };
`````