Rank most retaining instances for each class.

runtime/observatory/lib/src/elements/class_view.dart

Index: runtime/observatory/lib/src/elements/class_view.dart
===================================================================
--- runtime/observatory/lib/src/elements/class_view.dart	(revision 42649)
+++ runtime/observatory/lib/src/elements/class_view.dart	(working copy)
@@ -14,6 +14,7 @@
   @published Class cls;
   @observable ServiceMap instances;
   @observable int retainedBytes;
+  @observable ObservableList mostRetained;
   ClassViewElement.created() : super.created();
 
   Future<ServiceObject> eval(String text) {
@@ -26,6 +27,16 @@
           instances = obj;
         });
   }
+  
+  Future<ServiceObject> retainedToplist(var limit) {
+    return cls.isolate.fetchHeapSnapshot().then(
+        (HeapSnapshot snapshot) =>
+            Future.wait(snapshot.getMostRetained(classId: cls.vmCid,
+                                                 limit: 10))).then(
+        (List<ServiceObject> most) {
+            mostRetained = new ObservableList.from(most);
+        });
+  }
 
   // TODO(koda): Add no-arg "calculate-link" instead of reusing "eval-link".
   Future<ServiceObject> retainedSize(var dummy) {
@@ -37,6 +48,7 @@
   void refresh(var done) {
     instances = null;
     retainedBytes = null;
+    mostRetained = null;
     cls.reload().whenComplete(done);
   }