Fix class hierarchy display

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 {
         border-spacing: 0px;
         width: 100%;
         margin-bottom: 20px
-      vertical-align: middle;
+        vertical-align: middle;
       }
 
       tr {
         background-color: #FFFFFF;
       }
 
       tr:hover {
         background-color: #FAFAFA;
       }
+
+      th {
+        text-align: left;
+      }
     </style>
     <nav-bar>
       <top-nav-menu last="{{ true }}"></top-nav-menu>
     </nav-bar>
     <div class="content-centered">
       <h1>Class Hierarchy</h1>
       <table id="tableTree" class="table">
         <thead id="tableTreeHeader">
           <tr>
             <th>Class</th>
           </tr>
         </thead>
         <tbody id="tableTreeBody"></tbody>
       </table>
     </div>
   </template>
 </polymer-element>
 
 <script type="application/dart" src="class_tree.dart"></script>