Consider making `"a" as "b"` a compile error

**TypeScript Version:**  2.8.0-dev.20180223

**Search Terms:** string literal type assertion

**Code**

```ts
"a" as "b"
```

**Expected behavior:**

Error: type "a" can not be converted to type "b".

**Actual behavior:**

No error.

**Related Issues:**  #10195, which would make it so that no one would need to write `"a" as "a"`.
