Avoid to list internal Arrays in _GetRetainingPath api

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

 <link rel="import" href="../../../../packages/polymer/polymer.html">
 <link rel="import" href="error_view.html">
 <link rel="import" href="field_ref.html">
 <link rel="import" href="inbound_reference.html">
 <link rel="import" href="instance_ref.html">
 <link rel="import" href="eval_link.html">
 
 <polymer-element name="object-common">
   <template>
     <link rel="stylesheet" href="css/shared.css">
@@ skipping 47 matching lines @@
                        label="[find]"
                        expr="20">
             </eval-link>
           </template>
           <template if="{{ path != null }}">
             <template if="{{ path['length'] == 0 }}">
               &lt;root&gt;
             </template>
             <template repeat="{{ element in path['elements'] }}">
             <div class="memberItem">
-              <div class="memberName">[{{ element['index']}}]</div>
+              <div class="memberName">[{{ path['elements'].indexOf(element) }}]</div>
               <div class="memberValue">
                 <template if="{{ element['parentField'] != null }}">
                   from <any-service-ref ref="{{ element['parentField'] }}"></any-service-ref> of
                 </template>
                 <template if="{{ element['parentListIndex'] != null }}">
                   from [{{ element['parentListIndex'] }}] of
                 </template>
+                <template if="{{ element['parentMapKey'] != null }}">
+                  from [<any-service-ref ref="{{ element['parentMapKey'] }}"></any-service-ref>] of
+                </template>
                 <template if="{{ element['_parentWordOffset'] != null }}">
                   from word[{{ element['_parentWordOffset'] }}] of
                 </template>
                 <any-service-ref ref="{{ element['value'] }}"></any-service-ref>
               </div>
               </div>
             </template>
             <template if="{{ path['length'] > path['elements'].length }}">
               showing {{ path['elements'].length }} of {{ path['length'] }}
               <eval-link
@@ skipping 24 matching lines @@
             </template>
           </template>
         </div>
       </div>
 
    </div>
   </template>
 </polymer-element>
 
 <script type="application/dart" src="object_common.dart"></script>