application-route.js

export default Ember.Route.extend({
  beforeModel() {
    this.get('currentUser').fetchCurrentUser().then(()=>{
      // do stuff
    });
  }
})

current-user.js

import Ember from "ember";

export default Ember.Service.extend({
  store: Ember.inject.service('store:main'),
  _currentUser: null,
  unknownProperty: function(key) {
    return this.get('_currentUser').get(key);
  },
  fetchCurrentUser: function() {
    this.get('store').find('managedUser', 'current').then((user) => {
      this.set('_currentUser', user);
    });
  }
});

initializer.js

import Ember from "ember";;
import CurrentUserService from "frontend/services/current-user";

const initialize = function(registry, app) {
  app.register("user:current", CurrentUserService, { instantiate: true, singleton: true });
  ["route", "controller", "component"].forEach(function(item){
    app.inject(item, "currentUser", "user:current")
  });
};

export default { name: "current-user", after: "store", initialize: initialize };