Add persistent handles to service protocol and Observatory UI

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

 <link rel="import" href="../../../../packages/polymer/polymer.html">
 <link rel="import" href="action_link.html">
 <link rel="import" href="observatory_element.html">
 
 <polymer-element name="nav-bar" extends="observatory-element">
   <template>
     <link rel="stylesheet" href="css/shared.css">
     <style>
       nav {
         position: fixed;
@@ skipping 189 matching lines @@
       <nav-menu-item link="{{ makeLink('/profiler-table', isolate) }}"
                      anchor="cpu profile (table)"></nav-menu-item>
       <nav-menu-item link="{{ makeLink('/allocation-profiler', isolate) }}"
                      anchor="allocation profile"></nav-menu-item>
       <nav-menu-item link="{{ makeLink('/heap-map', isolate) }}"
                      anchor="heap map"></nav-menu-item>
       <nav-menu-item link="{{ makeLink('/metrics', isolate) }}"
                      anchor="metrics"></nav-menu-item>
       <nav-menu-item link="{{ makeLink('/heap-snapshot', isolate) }}"
                      anchor="heap snapshot"></nav-menu-item>
+      <nav-menu-item link="{{ makeLink('/persistent-handles', isolate) }}"
+                     anchor="persistent handles"></nav-menu-item>
       <nav-menu-item link="{{ makeLink('/ports', isolate) }}"
                      anchor="ports"></nav-menu-item>
       <nav-menu-item link="{{ makeLink('/logging', isolate) }}"
                      anchor="logging"></nav-menu-item>
       <content></content>
     </nav-menu>
   </template>
 </polymer-element>
 
 <polymer-element name="library-nav-menu" extends="observatory-element">
@@ skipping 230 matching lines @@
         [<a class="link" on-click="{{ goto }}"
             _href="{{ gotoLink('vm-connect') }}">Connect to a different VM</a>]
         <a class="boxclose" on-click="{{ closeItem }}">&times;</a>
     </div>
     </template>
   </template>
 </polymer-element>
 
 
 <script type="application/dart" src="nav_bar.dart"></script>