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

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

Index: runtime/observatory/lib/src/elements/object_common.html
diff --git a/runtime/observatory/lib/src/elements/object_common.html b/runtime/observatory/lib/src/elements/object_common.html
index 7cad87a8fb22666edd0a5ab158dd8e3a4422a750..14311e072e2263248b823cd2b0af8196ccc692e6 100644
--- a/runtime/observatory/lib/src/elements/object_common.html
+++ b/runtime/observatory/lib/src/elements/object_common.html
@@ -26,6 +26,20 @@
         <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">