Object.setPrototypeOf(O, proto) IS in ES6

This one is a short one, just to confirm that finally Object.setPrototypeOf(obj, proto) made it into ES6.
The section 15.2.3.2 speaks clearly:

15.2.3.2 Object.setPrototypeOf ( O, proto )
When the setPrototypeOf function is called with arguments O and proto, the following steps are taken:

1. If Type(O) is not Object, then throw a TypeError exception.
2. If Type(proto) is neither Object or Null, then throw a TypeError exception.
3. Let status be the result of calling the [[SetInheritance]] internal method of O with argument proto.
4. ReturnIfAbrupt(status).
5. If status is false, then throw a TypeError exception.
6. Return O.

Without going into details, the most basic polyfill woud be like this:

(function(O,s){
O[s]||(O[s]=function(o,p){o.__proto__=p;return o})
}(Object,'setPrototypeOf'));

If you want to go into details, the full reliable polyfill is hard to write down due all inconsistencies across JS engines out there where the __proto__ setter cannot be reused which means it's not possible to trust this magic over objects created from null.
All you need to do is forget __proto__ and use the suggested polyfill until the day __proto__ can simply disappear from those specs pages.
Enjoy!

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This is a full polyfill with some extra info you might want to analyze, whenever ployfill is strictly true or false.