Earlier quoted context omitted.
Actually, for JS engines this is a much smaller problem than for other parts of the web platform. There are two nasty warts I'm aware of: the "function inside if" mess, and the fact that the spec says enumeration order is undefined but actual web pages depend on some things about enumeration order that all browsers implement; chances are this will make it into the spec at some point. But by and large JS engines agree…
Enumeration order was indeed (finally) added to the ES2015 spec. Here's a link to the release candidate spec for the [[OwnPropertyKeys]] algorithm: https://people.mozilla.org/~jorendorff/es6-draft.html#sec-or...
[[OwnPropertyKeys]] is not invoked by a for-in loop. [[Enumerate]] is instead, and while it defines that the set of iterated names is the same as that returned by [[OwnPropertyKeys]] it explicitly says that order is not defined.