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

runtime/observatory/lib/src/heap_snapshot/heap_snapshot.dart

Index: runtime/observatory/lib/src/heap_snapshot/heap_snapshot.dart
diff --git a/runtime/observatory/lib/src/heap_snapshot/heap_snapshot.dart b/runtime/observatory/lib/src/heap_snapshot/heap_snapshot.dart
index 2d7abcd313aa5b095ab199b480d75026c3f93486..5f0bd3eb4d86e8b1585510839ce859a2b909b9f6 100644
--- a/runtime/observatory/lib/src/heap_snapshot/heap_snapshot.dart
+++ b/runtime/observatory/lib/src/heap_snapshot/heap_snapshot.dart
@@ -124,6 +124,8 @@ class HeapSnapshotDominatorNode implements M.HeapSnapshotDominatorNode {
   final S.Isolate isolate;
   S.HeapObject _preloaded;
 
+  bool get isStack => v.isStack;
+
   Future<S.HeapObject> get object {
     if (_preloaded != null) {
       return new Future.value(_preloaded);
@@ -159,6 +161,8 @@ class HeapSnapshotMergedDominatorNode
   final MergedObjectVertex v;
   final S.Isolate isolate;
 
+  bool get isStack => v.isStack;
+
   Future<S.HeapObject> get klass {
     return new Future.value(isolate.getClassByCid(v.vmCid));
   }