Can't cast array to array subtype

**TypeScript Version:**  2.7.0-dev.20171109

**Code**

```ts
interface I extends Array<number> { x: number }
const x = [] as I;
```

**Expected behavior:**

No error.

**Actual behavior:**

```
src/a.ts(2,11): error TS2352: Type 'never[]' cannot be converted to type 'I'.
  Property 'x' is missing in type 'never[]'.
```