Add a version of heap snapshots that use only fields and stack frames as roots and only include ins…

runtime/observatory/lib/src/repositories/top_retaining_instances.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file
 
 part of repositories;
 
 class TopRetainingInstancesRepository
     implements M.TopRetainingInstancesRepository {
   Future<Iterable<M.RetainingObject>> get(M.IsolateRef i, M.ClassRef c) async {
     S.Isolate isolate = i as S.Isolate;
     S.Class cls = c as S.Class;
     assert(isolate != null);
     assert(cls != null);
-    final raw = await isolate.fetchHeapSnapshot(true).last;
+    final raw = await isolate.fetchHeapSnapshot("VM", true).last;

[Cutch, 2016/11/18 20:40:31]

HeapSnapshotRoots.VM

     final snapshot = new HeapSnapshot();
     await snapshot.loadProgress(isolate, raw).last;
     return (await Future.wait(
             snapshot.getMostRetained(isolate, classId: cls.vmCid, limit: 10)))
         .map((object) => new S.RetainingObject(object));
   }
 }