Give fields a model class in the service library.

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

 <link rel="import" href="../../../../packages/polymer/polymer.html">
 <link rel="import" href="class_ref.html">
 <link rel="import" href="error_view.html">
 <link rel="import" href="eval_box.html">
 <link rel="import" href="eval_link.html">
 <link rel="import" href="field_ref.html">
 <link rel="import" href="function_ref.html">
 <link rel="import" href="inbound_reference.html">
 <link rel="import" href="instance_ref.html">
 <link rel="import" href="observatory_element.html">
@@ skipping 131 matching lines @@
         <template if="{{ instance.fields.isNotEmpty }}">
           fields ({{ instance.fields.length }})
           <curly-block expand="{{ instance.fields.length <= 8 }}">
             <div class="memberList">
               <template repeat="{{ field in instance.fields }}">
                 <div class="memberItem">
                   <div class="memberName">
                     <field-ref ref="{{ field['decl'] }}"></field-ref>
                   </div>
                   <div class="memberValue">
-                    <any-service-ref ref="{{ field['value'] }}"></any-service-ref>
+                    <any-service-ref ref="{{ field.value }}"></any-service-ref>
                   </div>
                 </div>
               </template>
             </div>
           </curly-block><br><br>
         </template>
 
         <template if="{{ instance.elements.isNotEmpty }}">
           elements ({{ instance.elements.length }})
           <curly-block expand="{{ instance.elements.length <= 8 }}">
@@ skipping 18 matching lines @@
       </div>
 
       <br><br><br><br>
       <br><br><br><br>
 
     </template>
   </template>
 </polymer-element>
 
 <script type="application/dart" src="instance_view.dart"></script>