fix(menu): menu focus is lost when menu-item has custom content

src/components/menu/js/menuServiceProvider.js

@@ -267,7 +267,10 @@ function MenuProvider($$interimElementProvider) {
             if (focusTarget) {
               break;
             }
-            if (child.firstElementChild && !child.firstElementChild.disabled) {
+            // Need to check the attribute as well since this might be a custom element whose
+            // disabled property is undefined.
+            if (child.firstElementChild && !child.firstElementChild.disabled &&
+                !child.firstElementChild.getAttribute('disabled')) {
               focusTarget = child.firstElementChild;
               break;
             }