Correct shadow behavior in 12.6.4-2 (bugzilla #1450)

Fixes [Bugzilla #1450](https://bugs.ecmascript.org/show_bug.cgi?id=1450).

The test was not checking the appropriate shadowing behavior. If I understand correctly, a non-enumerable property `obj.prop` must shadow an enumerable property in the [[prototype]], `obj.[[proto]].prop`, preventing `prop` from being included in the `for-in` loop.

It passes in FF 26, it does not pass in Chrome 32.