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

runtime/bin/vmservice/observatory/deployed/web/index.html

Index: runtime/bin/vmservice/observatory/deployed/web/index.html
diff --git a/runtime/bin/vmservice/observatory/deployed/web/index.html b/runtime/bin/vmservice/observatory/deployed/web/index.html
index 2f72a5e86831f699ff12a356fa1e300fc92e4a01..dced7d94d7aff3e9f539378167df6026b783eff4 100644
--- a/runtime/bin/vmservice/observatory/deployed/web/index.html
+++ b/runtime/bin/vmservice/observatory/deployed/web/index.html
@@ -928,7 +928,7 @@ hr {
         unexpected reference type <{{ ref.serviceType }}>
       </template>
 
-      <template if="{{ isPsuedoNull(ref) }}">
+      <template if="{{ isSentinel(ref) }}">
         <div title="{{ hoverText }}">{{ ref['valueAsString'] }}</div>
       </template>
 
@@ -14533,11 +14533,11 @@ hr {
     <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">
@@ -14711,6 +14711,7 @@ hr {
 
 
 
+
 <polymer-element name="json-view" extends="observatory-element">
   <template>
     <nav-bar>