Add RPC and UI for reachable size of an object / all instances of a class.

runtime/observatory/lib/src/elements/object_common.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="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">
 <link rel="import" href="nav_bar.html">
 <link rel="import" href="eval_link.html">
 
 <polymer-element name="object-common" extends="observatory-element">
   <template>
     <link rel="stylesheet" href="css/shared.css">
     <div class="memberList">
 
       <div class="memberItem">
         <div class="memberName">class</div>
         <div class="memberValue">
           <class-ref ref="{{ object.clazz }}"></class-ref>
         </div>
       </div>
 
       <div class="memberItem" title="Space for this object in memory">
         <div class="memberName">shallow size</div>
         <div class="memberValue">{{ object.size | formatSize }}</div>
       </div>
 
+      <div class="memberItem" title="Space reachable from this object, excluding class references">
+        <div class="memberName">reachable size</div>
+        <div class="memberValue">
+          <template if="{{ reachableBytes == null }}">
+            <eval-link callback="{{ reachableSize }}"
+                       label="[calculate]">
+            </eval-link>
+          </template>
+          <template if="{{ reachableBytes != null }}">
+            {{ reachableBytes | formatSize }}
+          </template>
+        </div>
+      </div>
+
       <div class="memberItem" title="Space that would be reclaimed if references to this object were replaced with null">
         <div class="memberName">retained size</div>
         <div class="memberValue">
           <template if="{{ retainedBytes == null }}">
             <eval-link callback="{{ retainedSize }}"
                        label="[calculate]">
             </eval-link>
           </template>
           <template if="{{ retainedBytes != null }}">
             {{ retainedBytes | formatSize }}
@@ skipping 61 matching lines @@
             </template>
           </template>
         </div>
       </div>
 
    </div>
   </template>
 </polymer-element>
 
 <script type="application/dart" src="object_common.dart"></script>