Consider updating https://github.com/andyearnshaw/Intl.js/blob/master/src/util.js#L17-L24 to test for accessor property support. Your use of defineProperty does define accessors, but your compat test doesn't verify that that will work. If Intl is loaded through the polyfill service, we polyfill defineProperty for you as a dependency, and our implementation throws in IE <=8 if you try to define an accessor property.

By testing the parts of the API that you want to use, you can make best use of any existing implementation, including a polyfill, before falling back to your naive implementation that ignores accessor properties where they are not supported.

Also see polyfillpolyfill/polyfill-service#712