code: function readOverlay(application) {
    var possibleValues, value;
    if (Ember.testing) {
      // when in test mode, use the overlay specified in the application
      value = {from: 'test:startApp()', name: application.get('devFixtures.overlay')};
    }
    else {
      // else try many locations
      possibleValues = [];
      location.href.replace(/(?:\?|&)FIXTURES_OVERLAY(?:=([^&#$]*))?(?:&|#|$)/, function (dummy, value) {
        value = value ? decodeURIComponent(value) : null;
        possibleValues.push({from: 'location.search:FIXTURES_OVERLAY', name: value});
      });
      possibleValues.push({
        from: 'file:config/environment.js',
        name: application.get('devFixtures.overlay')
      });
      possibleValues.push({
        from: 'localStorage:' + STORAGE_KEY,
        name: window.localStorage.getItem(STORAGE_KEY)
      });
      // grabbing the first one not undefined
      value = Ember.A(possibleValues).find(function (value) {
        return value !== undefined;
      });
      // saving it in the localStorage
      if (value && value.name) {
        window.localStorage.setItem(STORAGE_KEY, value.name);
      }
      else {
        window.localStorage.removeItem(STORAGE_KEY);
      }
    }
    // no overlay found anywhere
    if (!value) {
      value = {from: 'default', name: null};
    }
    return value;
  }
docs: Reads and save the overlay
  
  @function readOverlay
  @param {Ember.Application} application
  @return {{from: string, name: string|null}}