Use Polymer.dom with many dom manipulations.

tracing/tracing/ui/tracks/track.html

Index: tracing/tracing/ui/tracks/track.html
diff --git a/tracing/tracing/ui/tracks/track.html b/tracing/tracing/ui/tracks/track.html
index 09a54c272239818c748d6df7b59fda4d84527767..4b151ebf8f571a10a97679ef2f5f59399ac938b3 100644
--- a/tracing/tracing/ui/tracks/track.html
+++ b/tracing/tracing/ui/tracks/track.html
@@ -28,7 +28,7 @@ tr.exportTo('tr.ui.tracks', function() {
         throw new Error('viewport is required when creating a Track.');
 
       this.viewport_ = viewport;
-      this.classList.add('track');
+      Polymer.dom(this).classList.add('track');
     },
 
     get viewport() {
@@ -57,11 +57,11 @@ tr.exportTo('tr.ui.tracks', function() {
     context: function() {
       // This is a little weird here, but we have to be able to walk up the
       // parent tree to get the context.
-      if (!this.parentNode)
+      if (!Polymer.dom(this).parentNode)
         return undefined;
-      if (!this.parentNode.context)
+      if (!Polymer.dom(this).parentNode.context)
         throw new Error('Parent container does not support context() method.');
-      return this.parentNode.context();
+      return Polymer.dom(this).parentNode.context();
     },
 
     decorateChild_: function(childTrack) {