Unused parameter warning for type parameter in interface but not for type alias

**TypeScript Version:**  nightly (2.3.0-dev.20170415)

**Code**

```ts
type Alias<T> = { x: number };
interface I<T> { x: number; }
```

**Expected behavior:**

Both are `--noUnusedParameters` errors.

**Actual behavior:**

Only the interface is an error.