Add a new Sentinel type for sentinel responses. (They are not Null).

runtime/bin/vmservice/observatory/lib/src/elements/instance_view.html

 <link rel="import" href="../../../../packages/polymer/polymer.html">
 <link rel="import" href="class_ref.html">
 <link rel="import" href="error_view.html">
 <link rel="import" href="eval_box.html">
 <link rel="import" href="eval_link.html">
 <link rel="import" href="field_ref.html">
 <link rel="import" href="function_ref.html">
 <link rel="import" href="inbound_reference.html">
 <link rel="import" href="instance_ref.html">
 <link rel="import" href="observatory_element.html">
@@ skipping 12 matching lines @@
       <nav-control></nav-control>
     </nav-bar>
 
     <template if="{{ instance['error'] != null }}">
       <error-view error_obj="{{ instance['error'] }}"></error-view>
     </template>
 
     <template if="{{ instance['error'] == null }}">
       <div class="content">
         <!-- TODO(turnidge): Handle null instances. -->
-        <template if="{{ isType(instance.serviceType) }}">
-          <h1>type {{ instance['user_name'] }}</h1>
+        <template if="{{ isType(instance) }}">
+          <h1>type {{ instance.name }}</h1>
         </template>
-        <template if="{{ !isType(instance.serviceType) }}">
-          <h1>instance of {{ instance['class']['user_name'] }}</h1>
+        <template if="{{ !isType(instance) }}">
+          <h1>instance of {{ instance['class'].name }}</h1>
         </template>
         <div class="memberList">
           <div class="memberItem">
             <div class="memberName">class</div>
             <div class="memberValue">
               <class-ref ref="{{ instance['class'] }}">
               </class-ref>
             </div>
           </div>
           <template if="{{ instance['valueAsString'] != null }}">
@@ skipping 158 matching lines @@
       <div class="content">
         <eval-box callback="{{ eval }}"></eval-box>
       </div>
       <br><br><br><br>
       <br><br><br><br>
 
     </template>
   </template>
 </polymer-element>
 
-<script type="application/dart" src="instance_view.dart"></script>
+<script type="application/dart" src="instance_view.dart"></script>