bug: type predicates (`V is T`) fails on expressions (`V[K] is T`)

**TypeScript Version:** nightly (2.2.0-dev.20161128)

**Code**

```ts
propIs<T, V, K extends keyof V>(type: T, name: K, obj: V): V[K] is T;
```

**Expected behavior:**
TS being totally cool with `V[K] is T`.

**Actual behavior:**
Nope, many syntax errors.
