[BUGFIX release] Remove deprecation warning for dev, keep IE 8 compat

[sly7-7]

Closes #3465

cc/ @bmac @stefanpenner

[wecc]

Would it make sense to export create and keysFunc from a separate module and import instead of having Object.create || Ember.create / Object.keys || Ember.keys everywhere?

[sly7-7]

@wecc Not sure if it's worth it, but it maybe be good for further refactorings.

[bmac]

Looks good to me. I think the build failure might be fixed by 6f1a6f8. I can cherry pick that over to the release branch in the morning when I'm on my laptop.

[sly7-7]

@bmac I cherry picked your commit in that branch and force pushed. Now failing for a good reason.

[sly7-7]

@bmac, instead I've created #3478 with your fix, and also update tests here (we can't import modules in tests)