This was previously iterating the list of items and returning true if all items `isCompleted` property was true. 

Where is the `TodosController` getting `completed` from?

I think that it might be better to use [isEvery](http://emberjs.com/api/classes/Ember.Array.html#method_isEvery) (which essentially replaced `everyProperty` and `everyBy`).  This might look like:

```javascript
allAreDone: function() {
  return this.get('length') > 0 && this.isEvery('isCompleted', true)
}.property('@each.isCompleted')
```