[polymer] Switches .appendChild() to Polymer.dom()...appendChild()

tracing/tracing/ui/base/toolbar_button_test.html

Index: tracing/tracing/ui/base/toolbar_button_test.html
diff --git a/tracing/tracing/ui/base/toolbar_button_test.html b/tracing/tracing/ui/base/toolbar_button_test.html
index 78f9408116367ddebbe0e33993d002b4f693ad35..3cdd78eadf734e9facb51cf22d470713784097fd 100644
--- a/tracing/tracing/ui/base/toolbar_button_test.html
+++ b/tracing/tracing/ui/base/toolbar_button_test.html
@@ -25,7 +25,7 @@ tr.b.unittest.testSuite(function() {
     el.style.width = '100px';
     el.style.height = '40px';
 
-    el.appendChild(tr.ui.b.createSpan({textContent: 'blahblah'}));
+    Polymer.dom(el).appendChild(tr.ui.b.createSpan({textContent: 'blahblah'}));
 
     this.addHTMLOutput(el);
   });