Support class exports or clarify error message

One nice pattern I've used for Durable Objects is to implement the eyeball fetch as a static method on the object's class, ie:

```js
class Host {
static fetch(request, env) {
let {hostname} = new URL(request.url)
let hostId = env.hosts.idFromName(hostname)
return env.hosts.get(hostId).fetch(request)
}

fetch(request) {
return new Response('ok')
}
}
```

Currently I can use this pattern like this:

```js
export {Host}
export default {fetch: Host.fetch}
```

but ideally I'd be able to export the class itself, since it meets the requirements of being an object with a `fetch` property:

```js
export {Host, Host as default}
```

`workerd serve` accepts this, but then fails at runtime with the following error:

```
workerd/io/worker-entrypoint.c++:210: error: e = kj/async-io-unix.c++:1651: failed: connect() blocked by restrictPeers()
```

I feel like `workerd` should either support this kind of export, or fail at compile time with a clearer error.