Use Polymer.dom with many dom manipulations.

tracing/tracing/ui/base/dom_helpers_test.html

Index: tracing/tracing/ui/base/dom_helpers_test.html
diff --git a/tracing/tracing/ui/base/dom_helpers_test.html b/tracing/tracing/ui/base/dom_helpers_test.html
index 64ba68ba36fcb584a0ae04901bc8bf505b29b2a6..79720fb08931eaa0b54e7f3e85ec75140fe10762 100644
--- a/tracing/tracing/ui/base/dom_helpers_test.html
+++ b/tracing/tracing/ui/base/dom_helpers_test.html
@@ -22,8 +22,8 @@ tr.b.unittest.testSuite(function() {
       parent: divEl
     });
     var eltInDocument = document.querySelector('.a-div-class>.a-span-class');
-    assert.equal(eltInDocument.textContent, testText);
-    eltInDocument.parentElement.removeChild(eltInDocument);
+    assert.equal(Polymer.dom(eltInDocument).textContent, testText);
+    Polymer.dom(eltInDocument.parentElement).removeChild(eltInDocument);
   });
 
   test('createSpan_ownerDocument', function() {