wait for elements in other modules to be defined

test/unit/disable-upgrade.html

@@ -185,9 +185,10 @@ import '../../lib/mixins/disable-upgrade-mixin.js';
 suite('disable-upgrade', function() {
   let el;
 
-  setup(() => {
+  setup((done) => {
     el = document.createElement('my-element');
     document.body.appendChild(el);
+    window.addEventListener('DOMContentLoaded', function() { done(); });
   });
   teardown(function() {
     document.body.removeChild(el);