New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  
  emberVersion: Ember.VERSION,
  
  appName: '`selected` binding',
  
  choices: ['a', 'b', 'c', 'd'],
  
  selectedItem: 'a'
  
});

helpers.is-equal.js

import Ember from 'ember';

export function isEqual([l, r]) {
  return l === r;
}

export default Ember.Helper.helper(isEqual);

templates.application.hbs

<h1>{{appName}}</h1>
<p>Ember: {{emberVersion}}</p>
<select onchange={{action (mut selectedItem) value="target.value"}}>
  {{#each choices  as |choice|}}
    <option value={{choice}} selected={{is-equal value choice}}>{{choice}}</option>
  {{/each}}
</select>
<p>Selected: {{selectedItem}}</p>

twiddle.json

{
  "version": "0.10.5",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "release",
    "ember-data": "2.8.0",
    "ember-template-compiler": "release",
    "ember-testing": "release"
  },
  "addons": {}
}