ember: waiting until instance-initializer completes before doing anything else

I have an instance-initializer (below) for a shopping cart. The cart is an ember-data model because I am persisting to the back end. I am wanting all instances of my ember app to have a cart ID assigned before doing anything else.

I know the ideal is async, but item prices on the site are dependent on the cart. I am finding it would be easier to handle both the front end and the back end if I can depend on the cart always existing.

How can I tell ember to not do anything else until the cart instance-initializer completes and the ember-data cart model resolves?

My instance initializer:

export function initialize(appInstance) {
    let CartService = appInstance.factoryFor('service:cart');
    let cart = CartService.create();

    // check if there's a cart id present in localStorage
    if (window.localStorage) {
        if (window.localStorage.getItem('cart')) {
            //previous cart id found
            cart.loadCart(window.localStorage.getItem('cart'))
        } else {
            // no LS cart found, create a new one
            cart.createCart();
        }
    }

    appInstance.register('cart:main', cart, { instantiate: false });
    appInstance.inject('route', 'cart', 'cart:main');
    appInstance.inject('controller', 'cart', 'cart:main');
    appInstance.inject('component', 'cart', 'cart:main');
}

export default {
    name: 'cart',
    initialize
};

It handles two scenarios: a cart exists in localStorage, or no cart exists, create a new one. Those two methods are:

// cart service methods

loadCart(jsonCart) {
    // called from instance-initializer when a previous cart is found in localStorage

    let incomingCart = JSON.parse(jsonCart); // the raw cart in LS

    this.get('store').findRecord('cart', incomingCart.cartid).then(cart => {
        cart.setProperties(incomingCart);
        cart.save();
        set(this, 'cartObj', cart);
    });
},
createCart() {
    // called from instance-initializer when no prev cart is found

    let cart = this.get('store').createRecord('cart');
    cart.save().then(response => {
        window.localStorage.setItem('cart', JSON.stringify(response));
    });
    set(this, 'cartObj', cart);
}