Shadowed globals throw duplicate declaration error

The following code throws an error:
```js
const Worker = 'foo';
```
For reference, here's what the error looks like:
```
stdin:1: ERROR - Duplicate let / const / class / function declaration in the same scope is not allowed.
const Worker = 'foo';
^^^^^^^^^^^^^^

1 error(s), 0 warning(s)
```
The same error is producible with any other global. I'm pretty sure it's not an error to shadow globals in JS, regardless of the environment (i.e. it's part of the ECMAScript spec).

I think the culprit is in the way `externs` are defined. Since they're functions, the compiler treats this situation as a redeclaration.