Description

As we know, for..in is borken in IE. IE skips a property of an object if there's a DontEnum property (with the same name) in the prototype chain.

This results in all kinds of nasty stuff in IE:

Object.keys({ toString: true }).length; // => 0
Object.keys({ toString: false, valueOf: 'foo' }).length; // => 0

This also affects Class.Methods.addMethods though not at such extent - (toString and valueOf are explicitly copied over, but the other "buggy" ones - hasOwnProperty, isPrototypeOf, etc. are left over)