Working using PromiseObject to test with Fastboot

components.test-service.js

import Ember from 'ember';

export default Ember.Component.extend({
  slow: Ember.inject.service(),
  
  async init() {
    this._super(...arguments);
    
    const waitPromise = await this.get('slow.assetsPromise');
    
    console.log('init', waitPromise);
  }
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

services.slow.js

import Ember from 'ember';
import DS from 'ember-data';

export default Ember.Service.extend({
  assets: null,
  
  assetsPromise: Ember.computed(function() {
    let promise;
    
    if (this.assets) {
      console.log('already loaded');
      promise = new Ember.RSVP.Promise(resolve => {
        return resolve(this.assets);
      });
    } else {
      console.log('loading');
      promise = new Ember.RSVP.Promise(resolve => {
        return setTimeout(() => {
          return resolve("assets");
        }, 2000);
      }).then(result => {
        this.set('assets', result);
        return result;
      });
    }
    
    return DS.PromiseObject.create({ promise });
  })
});

templates.application.hbs

{{test-service title="first component"}}

{{test-service title="second component"}}

templates.components.test-service.hbs

{{#if slow.assetsPromise.isFulfilled}}
  {{title}}:
  {{slow.assetsPromise.content}}
{{/if}}

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}