WebKit's variable resolution order on window has caused a number of bugs for the Closure Compiler. See http://code.google.com/p/chromium/issues/detail?id=80591 for more details of the case that keeps biting them.
Also see http://lists.w3.org/Archives/Public/public-webapps/2011AprJun/0389.html and http://lists.w3.org/Archives/Public/public-script-coord/2012JanMar/0010.html for discussion on standardizing this. The discussion is not quite done, but it seems likely the WebKit/JSC/V8 will need to change.
I looked at a CL for this bug and yes it does require a v8 change for v8-based webkit browsers.
OK. This is resolved now. http://lists.w3.org/Archives/Public/public-script-coord/2012JanMar/0104.html
V8 has fixed this (behind a flag) but since all WebIDL attributes are implemented as own properties on the global object we need to fix bug 49739 to get non configurable properties to work correctly.
The ES5.2 behavior is at https://bugs.ecmascript.org/show_bug.cgi?id=78
Chrome is now shipping with this behavior.