Unknown Property

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  unknownProperty(key) {
    if (key === 'foo') {
      return 'i am foo';
    }
    
    return undefined;
  }
});

templates.application.hbs

<div>
	Value of <code>foo</code>: {{foo}}
</div>
<div>
  Value of <code>bar</code>: {{bar}}
</div>

twiddle.json

{
  "version": "0.11.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.10.2",
    "ember-data": "2.11.0",
    "ember-template-compiler": "2.10.2",
    "ember-testing": "2.10.2"
  },
  "addons": {}
}