Suppress duplicate variable warnings for namespaces

``` js
export enum Test {
  a, b, c
}

export module Test {
  export function abc() {
    return 1;
  }
}
```

becomes

``` js
...enum stuff...
var Test = exports.Test;
var Test;
...module stuff...
```

which is a duplicate variable error.

We could turn it off locally with

https://github.com/google/closure-compiler/wiki/@suppress-annotations
