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

[Domenic Denicola]

From: Mark S. Miller [mailto:erights at google.com] 

> In this situation, it will try and succeed. This more closely obeys the intent in the original code (e.g., the comment in the jQuery code), since it creates a non-configurable property on the *Window* W. It does not violate any invariant, since all that's observable on the *WindowProxy* (given the rest of your draft spec, which remain unchanged) is a configurable property of the same name.

Ah, I see! So then another non-intuitive (but invariant-preserving) consequence would be:

```js
Object.defineProperty(window, "prop", { value: "foo" });

var propDesc = Object.getOwnPropertyDescriptor(window, "prop");

if (propDesc.configurable) {
  Object.defineProperty(window, "prop", { value: "bar" });

  // this will fail, even though the property is supposedly configurable,
  // since when it forwards from the WindowProxy `window` to the underlying
  // Window object, it the Window's [[DefineOwnProperty]] fails.
}
```

Am I getting this right?