Rank most retaining instances for each class.

runtime/observatory/lib/object_graph.dart

Index: runtime/observatory/lib/object_graph.dart
===================================================================
--- runtime/observatory/lib/object_graph.dart	(revision 42649)
+++ runtime/observatory/lib/object_graph.dart	(working copy)
@@ -38,6 +38,9 @@
 class ObjectVertex {
   // Never null. The isolate root has id 0.
   final int _id;
+  bool get isRoot => _id == 0;
+  // TODO(koda): Include units in object graph metadata.
+  int addressForWordSize(int bytesPerWord) => _id * 2 * bytesPerWord;
   // null for VM-heap objects.
   int _shallowSize;
   int get shallowSize => _shallowSize;
@@ -75,12 +78,27 @@
       _addFrom(reader);
     }
     _computeRetainedSizes();
+    _mostRetained = new List<ObjectVertex>.from(
+        vertices.where((u) => !u.isRoot));
+    _mostRetained.sort((u, v) => v.retainedSize - u.retainedSize);
   }
 
   Iterable<ObjectVertex> get vertices => _idToVertex.values;
+  List<ObjectVertex> _mostRetained;
 
   ObjectVertex get root => _asVertex(0);
   
+  Iterable<ObjectVertex> getMostRetained({int classId, int limit}) {
+    var result = _mostRetained;
+    if (classId != null) {
+      result = result.where((u) => u.classId == classId);
+    }
+    if (limit != null) {
+      result = result.take(limit);
+    }
+    return result;
+  }
+  
   void _computeRetainedSizes() {
     // The retained size for an object is the sum of the shallow sizes of
     // all its descendants in the dominator tree (including itself).