to check or not to check

controllers.application.js

import Ember from 'ember';

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

helpers.eq.js

import Ember from 'ember';

export function eq([a, b]) {
  return a === b;
}

export default Ember.Helper.helper(eq);

templates.application.hbs

<h1>checked = {{input value=checked}}</h1>
<br>
<br>
<input type="radio" name="flex" checked={{eq checked "checked"}}> checked when field is "checked"

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}