Route#prefetch & Route#prefetched

prefetch.js

App.PostRoute = Ember.Route.extend({
  prefetch(params) {
    return this.store.findRecord('post', params.id);
  }
});

App.PostCommentsRoute = Ember.Route.extend({
  prefetch(params, transition) {
    return this.store.findAll('comments', transition.params.post.id);
  },
  
  async model() {
    return {
      OP: (await this.prefetched('post')).author,
      comments: await this.prefetched(this.routeName)
    };
  }
]);

Route#prefetch hook loads data in parallel.

Route#prefetched method gets the promise returned from Route#prefetch for a given route name.

Default Route#model hook returns the promise returned by Route#prefetch if it exists, otherwise performs the existing default.

Can almost be accomplished with Router#willTransition public API, a reserved property on Route (I call it asyncData, but that could be changed), and one reopen:

1. Add a willTransition subscriber to router:main, which synchronously calls all Route#prefetch hooks for the transition, storing the result at Route#asyncData. (Currently, broken. Need a willTransition like thing that also fires on transition redirects.)
2. Reopen Route to add Route#prefetched method and extend default Route#model hook.