Imported const enum is not inlined in generated code

**TypeScript Version:**  2.4.0

**Code**

```ts
// a.ts
export const enum Foo {
  Bar = 'bar'
}

// b.ts
import { Foo } from './a'

Foo.Bar // TypeError or ReferenceError, depending on how it's compiled
```

**Expected behavior:**
`Foo.Bar` to be replaced with `'bar'`

**Actual behavior:**
The import is deleted and `Foo.Bar` is left as is