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

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

 <link rel="import" href="../../../../packages/polymer/polymer.html">
 <link rel="import" href="curly_block.html">
 <link rel="import" href="observatory_element.html">
 <link rel="import" href="service_ref.html">
 
 <polymer-element name="instance-ref" extends="service-ref">
   <template>
     <link rel="stylesheet" href="css/shared.css">
     <style>
       .errorBox {
         background-color: #f5f5f5;
         border: 1px solid #ccc;
         padding: 10px;
         font-family: consolas, courier, monospace;
         font-size: 1em;
         line-height: 1.2em;
         white-space: pre;
       }
     </style>
     <span>
       <template if="{{ isError(ref) }}">
          <pre class="errorBox">{{ ref.message }}</pre>
       </template>
 
       <template if="{{ isUnexpected(ref) }}">
         unexpected reference type &lt;{{ ref.serviceType }}&gt;
       </template>
 
-      <template if="{{ isPsuedoNull(ref) }}">
+      <template if="{{ isSentinel(ref) }}">
         <div title="{{ hoverText }}">{{ ref['valueAsString'] }}</div>
       </template>
 
       <template if="{{ (isString(ref) ||
                         isBool(ref) ||
                         isNull(ref) ||
                         isInt(ref)) ||
                         isDouble(ref)) }}">
         <a on-click="{{ goto }}" href="{{ url }}">{{ ref['valueAsString'] }}</a>
       </template>
@@ skipping 42 matching lines @@
               </div>
             </template>
           </div>
         </curly-block>
       </template>
     </span>
   </template>
 </polymer-element>
 
 <script type="application/dart" src="instance_ref.dart"></script>