PSR-4 ify the WC_Admin_Order_Trait class.

plugins/woocommerce-admin/src/WC_Admin_Order.php

@@ -18,7 +18,7 @@ class WC_Admin_Order extends \WC_Order {
 	/**
 	 * Order traits.
 	 */
-	use \WC_Admin_Order_Trait;
+	use \Automattic\WooCommerce\Admin\WC_Admin_Order_Trait;
 
 	/**
 	 * Holds refund amounts and quantities for the order.

plugins/woocommerce-admin/src/WC_Admin_Order_Refund.php

@@ -18,7 +18,7 @@ class WC_Admin_Order_Refund extends \WC_Order_Refund {
 	/**
 	 * Order traits.
 	 */
-	use \WC_Admin_Order_Trait;
+	use \Automattic\WooCommerce\Admin\WC_Admin_Order_Trait;
 
 	/**
 	 * Add filter(s) required to hook WC_Admin_Order_Refund class to substitute WC_Order_Refund.

plugins/woocommerce-admin/src/WC_Admin_Order_Trait.php

@@ -7,6 +7,8 @@
  * @package WooCommerce Admin/Classes
  */
 
+namespace Automattic\WooCommerce\Admin;
+
 defined( 'ABSPATH' ) || exit;
 
 /**