Can't use implements with an unnamed class expression without using extends

``` ts
interface Foo {
    doThing(): void;
}

let cls = class implements Foo {
    doThing() { }
};
```

TS errors on this because it thinks `implements` is supposed to be the class name for the class expression, rather than the start of an `implements` clause.
This is fine, however:

``` ts
let cls = class extends null implements Foo {
    doThing() { }
};
```
