Regression: Unable to override statics of class

```js
declare class Foo extends Array{
static bind(): Foo;
}
```

This makes it impossible to provide a flow libdef for Bluebird:

```js
declare class Bluebird$Promise<+R> extends Promise{
static bind(ctx: any): Bluebird$Promise;
}
```

http://bluebirdjs.com/docs/api/promise.bind.html