Exported contants under namespace not reassignable

```ts
export namespace ns {
  export const a: i32 = 1;
  export const b: i32 = a;
}
```
Output:
```
ERROR TS2304: Cannot find name 'a'.

   export const b: i32 = a;
                         ~
```
But this works:
```ts
export namespace ns {
  export const a: i32 = 1;
  export const b: i32 = ns.a;
}
```