io.js, ES6: TypeError: Class constructors cannot be invoked without 'new'

When trying to run server-side using io.js (2.3.3) using es6 classes i get the following error:

```
/Users/josef/Develop/some/some-app/src/js/shared/stores/some-store.js:43
constructor() {
^
TypeError: Class constructors cannot be invoked without 'new'
at Store.SomeStore (/Users/josef/Develop/some/some-app/src/js/shared/stores/some-store.js:43:14)
at new Store (/Users/josef/Develop/some/some-app/node_modules/alt/lib/store/index.js:139:73)
at Object.createStoreFromClass (/Users/josef/Develop/some/some-app/node_modules/alt/lib/store/index.js:160:15)
at Alt.createStore (/Users/josef/Develop/some/some-app/node_modules/alt/lib/index.js:116:109)
...
```

From a store in this format:

```
'use strict';
var alt = require('../alt'),
someActions = require('../actions/some-actions'):

class SomeStore {
constructor() {

this.state = {};

this.bindListeners({
onSomeCreated: someActions.someCreated
});
}

onSomeCreated(some) {
}
}

module.exports = alt.createStore(SomeStore, 'SomeStore');
```

Is there some configuration that should be set for alt to know how initialisation should happen? Or could I be missing some other obvious thing?

Start with the reproduced stack trace in src/js/shared/stores/some-store.js and trace the createStore and createStoreFromClass calls shown in alt/lib/store/index.js. Determine whether io.js 2.3.3 requires configuration or a supported store-class initialization path, then verify the result against the reported ES6 class example.

Written by the indexing model from the issue text.