ArrayProxy does not have `concat`

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  proxy: Ember.computed(() => Ember.ArrayProxy.create({ content: [1, 2, 3] })),
  
  concatProxy: Ember.computed('proxy', function () {
    // Uncomment this and it breaks
  	// return this.get('proxy').concat([4, 5, ,6]);
    
    return this.get('proxy');
  }),
  
  array: Ember.computed(() => [1, 2, 3]),
  
  concatArray: Ember.computed('array', function () {
  	return this.get('array').concat([4, 5, ,6]);
  }),
  
  testing: 'lalala'
});

templates.application.hbs

<div>
	Array: 
	{{#each array as |item|}}
		{{item}}
	{{/each}}
</div>

<div>
	ConcatArray: 
	{{#each concatArray as |item|}}
		{{item}}
	{{/each}}
</div>

<div>
	Proxy: 
	{{#each proxy as |item|}}
		{{item}}
	{{/each}}
</div>

<div>
	ConcatProxy: 
	{{#each concatProxy as |item|}}
		{{item}}
	{{/each}}
</div>

This won't be rendered properly: {{testing}}

twiddle.json

{
  "version": "0.11.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.11.0",
    "ember-data": "2.11.0",
    "ember-template-compiler": "2.11.0",
    "ember-testing": "2.11.0"
  },
  "addons": {}
}