Tweaks to make Cesium not totally bomb on IE8.

In celebration of the end of life of Windows XP, here's a pull request that fixes the few problems that prevent Cesium modules from running to completion on IE8.  Of course, if you actually try to use the modules, pretty much nothing will work, but you weren't going to do that anyway, were you?  You're just trying to include your Cesium.js script tag and prevent the world from ending long enough to inform the user that their browser is hopelessly old.  This pull request will help you do that.

I don't think there's anything here worth mentioning in `CHANGES.md`.