Give instances their own model class; move DartErrors out of instance-ref into their own error-ref.

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

 <link rel="import" href="../../../../packages/polymer/polymer.html">
 <link rel="import" href="curly_block.html">
 <link rel="import" href="function_ref.html">
 <link rel="import" href="instance_ref.html">
 <link rel="import" href="observatory_element.html">
 <link rel="import" href="script_ref.html">
 
 <polymer-element name="stack-frame" extends="observatory-element">
   <template>
     <link rel="stylesheet" href="css/shared.css">
     <div class="flex-row">
       <div class="flex-item-fixed-1-12">
       </div>
       <div class="flex-item-fixed-1-12">
         #{{ frame['depth'] }}
       </div>
       <div class="flex-item-fixed-9-12">
         <function-ref ref="{{ frame['function'] }}"></function-ref>
           ( <script-ref ref="{{ frame['script'] }}"
                         pos="{{ frame['tokenPos'] }}">
           </script-ref> )
 
           <curly-block>
             <div class="memberList">
               <template repeat="{{ v in frame['vars'] }}">
                 <div class="memberItem">
                   <div class="memberName">{{ v['name']}}</div>
                   <div class="memberValue">
-                    <instance-ref ref="{{ v['value'] }}"></instance-ref>
+                    <any-service-ref ref="{{ v['value'] }}"></any-service-ref>
                   </div>
                 </div>
               </template>
             </div>
           </curly-block>
       </div>
       <div class="flex-item-fixed-1-12">
       </div>
     </div>
   </template>
 </polymer-element>
 
 <script type="application/dart" src="stack_frame.dart"></script>