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

runtime/observatory/lib/src/models/repositories/heap_snapshot.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 models;
 
 enum HeapSnapshotLoadingStatus { fetching, loading, loaded }
 
 bool isHeapSnapshotProgressRunning(HeapSnapshotLoadingStatus status) {
   switch (status) {
@@ skipping 12 matching lines @@
 abstract class HeapSnapshotLoadingProgress {
   HeapSnapshotLoadingStatus get status;
   String get stepDescription;
   double get progress;
   Duration get fetchingTime;
   Duration get loadingTime;
   HeapSnapshot get snapshot;
 }
 
 abstract class HeapSnapshotRepository {
-  Stream<HeapSnapshotLoadingProgressEvent> get(IsolateRef isolate,
-      {bool gc: false});
+  Stream<HeapSnapshotLoadingProgressEvent> get(
+      IsolateRef isolate,
+      {HeapSnapshotRoots roots: HeapSnapshotRoots.vm,
+      bool gc: false});
 }