Improve performance of table trees by around 3x

runtime/observatory/lib/src/elements/class_tree.html

 <link rel="import" href="../../../../packages/polymer/polymer.html">
 <link rel="import" href="nav_bar.html">
 <link rel="import" href="observatory_element.html">
 <link rel="import" href="class_ref.html">
 
 <polymer-element name="class-tree" extends="observatory-element">
   <template>
     <link rel="stylesheet" href="css/shared.css">
     <style>
       .table {
@@ skipping 79 matching lines @@
         color: #ffffff;
       }
     </style>
     <nav-bar>
       <top-nav-menu last="{{ true }}"></top-nav-menu>
       <nav-control></nav-control>
     </nav-bar>
     <div class="content-centered">
       <h1>Class Hierarchy</h1>
       <table id="tableTree" class="table">
-        <thead>
+        <thead id="tableTreeHeader">
           <tr>
             <th>Class</th>
           </tr>
         </thead>
-        <tbody>
-          <tr template repeat="{{row in tree.rows }}">
-            <td on-click="{{toggleExpanded}}"
-                class="{{ coloring(row) }}"
-                style="{{ padding(row) }}">
-              <span id="expand" style="{{ row.expanderStyle }}">{{ row.expander }}</span>
-              <class-ref ref="{{ row.cls }}"></class-ref>
-            </td>
-          </tr>
-        </tbody>
+        <tbody id="tableTreeBody"></tbody>
       </table>
     </div>
   </template>
 </polymer-element>
 
 <script type="application/dart" src="class_tree.dart"></script>