Ember.A seems to modify array instance

We’re seeing a bug in the Ember Simple Auth dummy app (simplabs/ember-simple-auth#1260) that seems to be caused by the fact that ~Enumerable.uniq~Ember.A tries to modify the provided array. This only seems to affect addon projects where prototype extension is disabled (this is mostly an assumption for now though).

Reproduction steps:

var array = ['a', 'a'];
Object.freeze(array);
Ember.A(array);

// =>
ember.debug.js:21542 Uncaught TypeError: Cannot add property nextObject, object is not extensible
    at Object.defineProperty (ember.debug.js:21542)
    at applyMixin (ember.debug.js:20576)
    at MixinPrototype.apply (ember.debug.js:20770)
    at Object.A (ember.debug.js:37397)
    at eval (eval at init (query-string.js:35), <anonymous>:1:7)
    at Class.init (query-string.js:35)
    at new Class (ember.debug.js:36118)
    at Function._ClassMixinProps.create (ember.debug.js:36306)
    at Class.buildQueryString (oauth2-code.js:110)
    at Class.buildUrl (oauth2-code.js:120)

This only seems to fail in addon dummy apps but not in real apps.

/cc @Turbo87

[stefanpenner]

Ya. It’s already mega costly. Copying would also likely break some code that expects the array to be The same.

Work should be done so that Ember.A can be phased out

[marcoow]

I guess the fix for actually making it so that the object wasn’t modified at all would be to always clone the object before applying the mixin (if prototype extensions are disabled/the mixin hasn’t already been applied on the prototype) but I guess the performance overhead for that is prohibitive?