WooCommerce – How to get shipping class of cart items in calculate_shipping function?

I have created a WooCommerce plugin that enables free shipping for subscribers. It seems to have broken following a recent WooCommerce upgrade.

Specifically, the problem seems that the shipping class of the cart items may not be being retrieved correctly.

Here is my calculate_shipping code – can anyone advise what is wrong?

/**
 * Add free shipping option for customers in base country with an active subscription,
 * but only if the cart doesn't contain an item with the 'heavy-item-shipping-class'.
 *
 * @access public
 * @param mixed $package
 * @return void
 */
public function calculate_shipping($package) {

    global $woocommerce;

    // Check country and subscription status
    if (is_user_in_base_country() && does_user_have_active_subscription()) {

        // This flag will be set to TRUE if cart contains heavy items
        $disable_free_shipping = FALSE;

        // Get cart items
        $cart_items = $package['contents'];

        // Check all cart items
        foreach ($cart_items as $cart_item) {

            // Get shipping class
            $shipping_class = $cart_item['data']->shipping_class; // *** IS THIS THE RIGHT WAY TO GET THE SHIPPING CLASS ??? ***

            // If heavy item, set flag so free shipping option is not made available
            if ($shipping_class === 'heavy-item-shipping-class') {

                // Set flag
                $disable_free_shipping = TRUE;

                // Enough
                break;

            }

        }

        // If appropriate, add the free shipping option
        if ($disable_free_shipping === FALSE) {

            // Create the new rate
            $rate = array(
                'id' => $this->id,
                'label' => "Free Shipping",
                'cost' => '0',
                'taxes' => '',
                'calc_tax' => 'per_order'
            );

            // Register the rate
            $this->add_rate($rate);

        }
        else {

            // Doesn't qualify for free shipping, so do nothing

        }

    }

}

UPDATE
I’ve looked at the %package array and noticed that it now contains the shipping class under [shipping_class:protected]. (Previously, this must have been [shipping_class].) Is it possible to extract this data? If not, what is the correct way of doing it?