[Meteor 3] Meteor.callAsync doesn't provide result from server

To reproduce:
In a new app from Meteor 3.0-alpha.18, add this code to both the client and server:

```js
if (Meteor.isServer) {
  Meteor.methods({
    'server-only'() {
      return 'result'
    }
  });
} else {
  Meteor.callAsync('server-only').then(console.log);
}
```

Instead of logging `result`, it logs:
```
{stubValuePromise: Promise}
```