Currently all async ops are polled lazily, which means that op
initialization code is postponed until control is yielded to the event
loop. This has some weird consequences, e.g.
```js
let listener = Deno.listen(...);
let conn_promise = listener.accept();
listener.close();
// `BadResource` is thrown. A reasonable error would be `Interrupted`.
let conn = await conn_promise;
```
JavaScript promises are expected to be eagerly evaluated. This patch
makes ops actually do that.
