Allows to mutate `const` with non-null assertion

**TypeScript Version:**  2.8.0-dev.20180209

**Code**

```ts
declare const x: number | undefined;
x!++;
```

**Expected behavior:**

```
src/a.ts(2,2): error TS2540: Cannot assign to 'x' because it is a constant or a read-only property.
```

**Actual behavior:**

No error.