Biff and ES6

[Tackyshot]

admittedly, I'm newer to the syntax of ES6 and ESNext but I am curious as to what Biff's syntax would be for creating stores?

export class SomeStore extends Biff.createStore{
  _store = [];
  addStoreItem(item){
    this._store.push(item);
  }
  //etc...
}

That is the equivalent of what I have so far, but I can't seem to figure out how to add the dispatcher handler function to the class. Am I way off base and using class incorrectly, or does Biff not have support for ES6?