Bug: Can't observe property in inner component

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName:'Ember Twiddle'
});

application.template.hbs

{{outer-component}}

inner-component.component.js

import Ember from 'ember';

export default Ember.Component.extend({
  test: null,
  
  testObserver: Ember.observer('test', function() {
    alert('test');
  })
  
});

inner-component.template.hbs

{{#each test as |el|}}
  {{el}}
{{/each}}

outer-component.component.js

import Ember from 'ember';

export default Ember.Component.extend({
  elements: ['1', '2', '3'],
  
  actions: {
  	addElement() {
    	this.get('elements').pushObject('test')  
    }
  }
});

outer-component.template.hbs

<h2>Press the button, alert should be triggered</h2>
{{inner-component test=elements}}

<button {{action 'addElement'}}>Add</button>

twiddle.json

{
  "version": "0.4.13",
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/1.13.10/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/1.13.13/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/1.13.10/ember-template-compiler.js"
  }
}