Name clash with CSS extern

We were trying to make a "Rect" class, but Rect is defined in the css externs file:

```
ERROR - Variable Rect first declared in externs.zip//browser/w3c_css.js
```

However, Rect is not available in JS (at least not in Chrome). So this basically means that Closure is banning `Rect` from ever being used. Shouldn't the externs that aren't actually available as JS objects be obfuscated in some way so that they are less likely to clash with client code? E.g. W3c_Css_Rect or something like that?

Not sure if this is a problem or if we just don't understand how this is supposed to work.