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

tracing/tracing/core/test_utils.html

Index: tracing/tracing/core/test_utils.html
diff --git a/tracing/tracing/core/test_utils.html b/tracing/tracing/core/test_utils.html
index 82e71efdf600479d255f0b4e45aacdaf73dc58ec..6f79d3d3d1d38bcf8b461650923302261cd69b9a 100644
--- a/tracing/tracing/core/test_utils.html
+++ b/tracing/tracing/core/test_utils.html
@@ -443,8 +443,8 @@ tr.exportTo('tr.c', function() {
     });
 
     var outputEl = document.createElement('div');
-    outputEl.appendChild(copyButtonEl);
-    outputEl.appendChild(testSourceEl);
+    Polymer.dom(outputEl).appendChild(copyButtonEl);
+    Polymer.dom(outputEl).appendChild(testSourceEl);
     test.addHTMLOutput(outputEl);
   };