New Twiddle

controllers.application.js

import Ember from 'ember';

const foo = Ember.computed({
  get() { return 'foo'; },
  set(key, value) { return value; },
});

class MyClass {}

Ember.defineProperty(MyClass.prototype, 'foo', foo);
                     
let instance = new MyClass();

Ember.set(instance, 'foo', 'bar');

console.log(instance.foo, Object.keys(instance));

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

templates.application.hbs

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

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}