override method of instance

controllers.application.js

import Ember from 'ember';

let Animal = Ember.Object.extend({
  bark() {
    return 'aurf!';
  }
});

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  init() {
    let animal = Animal.create();
    animal.bark = () => {
      return 'squeek!';
    };
    this.set('result', animal.bark());
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{result}}
<br>
<br>

twiddle.json

{
  "version": "0.7.2",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.4.4/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.4.3/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.4.4/ember-template-compiler.js"
  }
}