Allow abstract class for local declarations and expressions

**TypeScript Version:** 

1.8.9 / nightly (1.9.0-dev.20160612-1.0)

**Code**

``` ts
(() => {
    // works:
    class SomeClass { }

    // emits "error TS1184: Modifiers cannot appear here":
    abstract class SomeAbstractClass { }
})();
```

**Expected behavior:**
It should be possible to define abstract classes within functions just like non-abstract ones.
