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

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

 <link rel="import" href="../../../../packages/polymer/polymer.html">
 <link rel="import" href="instance_ref.html">
+<link rel="import" href="error_ref.html">
 
 <polymer-element name="eval-link">
   <template>
     <style>
       .idle {
         color: #0489c3;
         cursor: pointer;
       }
       .busy {
         color: #aaa;
         cursor: wait;
       }
     </style>
 
     <template if="{{ busy }}">
       <span class="busy">{{ label }}</span>
     </template>
     <template if="{{ !busy }}">
       <span class="idle"><a on-click="{{ evalNow }}">{{ label }}</a></span>
     </template>
     <template if="{{ result != null }}">
-      = <instance-ref ref="{{ result }}"></instance-ref>
+      = <any-service-ref ref="{{ result }}"></any-service-ref>
     </template>
   </template>
 </polymer-element>
 
 <script type="application/dart" src="eval_link.dart"></script>