Ember Data isNew does not change

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName:'Ember Twiddle',
  
  init() {
    this._super(...arguments);
    $.mockjax({
      url: '/posts',
      responseText: { 'data': { 'id': '1', 'type': 'post' } }
    });
  },
  
  newPost: Ember.computed(function() {
    return this.store.createRecord('post', { name: 'foo' });
  }),
  
  newPostIsNewChanged: Ember.observer('newPost.isNew', function() {
    console.log('newPost.isNew changed');
  }),
  
  newPostIdChanged: Ember.observer('newPost.id', function() {
    console.log('newPost.id changed');
  }),
  
  actions: {
    saveNewPost() {
      let post = this.get('newPost');
      post.save().then(function() {
        console.log('newPost saved');
      });
    }
  }
});

application.template.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>
<button {{action "saveNewPost"}}>{{newPost.name}}</button>

models.post.js

import DS from 'ember-data';

export default DS.Model.extend({
  name: DS.attr()
});

twiddle.json

{
  "version": "0.4.17",
  "EmberENV": {
    "FEATURES": {}
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.2.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember-template-compiler.js",
    "jquery-mockjax": "https://cdnjs.cloudflare.com/ajax/libs/jquery-mockjax/1.6.2/jquery.mockjax.js"
  }
}