Cannot re-export global symbol - circular definition of import alias

Assuming these two files:

`a.d.ts`:

``` ts
declare class X {}
```

`b.ts`:

```
/// <reference path="a.d.ts"/>
export {X};
```

Gives this error:

```
b.ts (2,9): Circular definition of import alias 'X'.
```

The use case for this is moving from internal modules to external modules by re-exporting symbols.

/cc @mhegazy & @rbuckton
