Fix typo

Author: Steven Orvell

test/unit/polymer-dom.html

@@ -527,7 +527,7 @@
     document.body.appendChild(el);
     const testEl = el.$.first;
     const testElDom = dom(testEl);
-    testEl.className = 'foo';
+    testElDom.className = 'foo';
     assert.isTrue(testEl.classList.contains('foo'));
     if (window['ShadyDOM']) {
       assert.isTrue(testEl.classList.contains('style-scope'));