Figuring out the behavior of WindowProxy in the face of non-configurable properties

[Domenic Denicola]

From: Travis Leithead [mailto:travis.leithead at microsoft.com] 

> WindowProxies are going to be a challenge when it comes to a pure ES implementation.

Shouldn't be too hard to implement a Direct Proxy that follows this proto-spec: https://github.com/domenic/window-proxy-spec