Ember.A().slice() with Array prototype extended

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  type: Ember.computed(function () {
    // it's function
    return typeof(Ember.A().slice().setEach);
  })
});

templates.application.hbs

{{type}}

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "EXTEND_PROTOTYPES": {
        "Array": true
    },
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.17.0",
    "ember-template-compiler": "2.17.0"
  }
}