New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  array: Ember.A(),
  
  actions: {
    addItem() {
      this.get('array').pushObject(Date.now());
    },
    mutateFirstItem() {
      const array = this.get('array');
      array.arrayContentWillChange(0, null, null);
      array.set('0', `mutated - ${Date.now()}`);
      array.arrayContentDidChange(0, null, null);
    }
  }
});

templates.application.hbs

<button onclick={{action "addItem"}}>addItem</button>
<button onclick={{action "mutateFirstItem"}}>mutateFirstItem</button>

{{array.length}}

<ul>
  {{#each array key="@index" as |item index|}}
    <li>{{index}} - {{item}}</li>
  {{/each}}
</ul>

twiddle.json

{
  "version": "0.10.6",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.9.0",
    "ember-data": "2.9.0",
    "ember-template-compiler": "2.9.0",
    "ember-testing": "2.9.0"
  },
  "addons": {}
}