Fix for issue 23647 (https://github.com/dart-lang/sdk/issues/23647)

runtime/vm/snapshot.h

Index: runtime/vm/snapshot.h
diff --git a/runtime/vm/snapshot.h b/runtime/vm/snapshot.h
index ef0777735cf856708da1567134b8b3598c495afd..669c1b707bffad90ab5aa63d7b86e7a913840257 100644
--- a/runtime/vm/snapshot.h
+++ b/runtime/vm/snapshot.h
@@ -626,6 +626,12 @@ class ForwardList {
   // Returns the id for the added object.
   intptr_t MarkAndAddObject(RawObject* raw, SerializeState state);
 
+  // Returns the id for the added object without marking it.
+  intptr_t AddObject(RawObject* raw, SerializeState state);
+
+  // Returns the id for the object it it exists in the list.
+  intptr_t FindObject(RawObject* raw);
+
   // Exhaustively processes all unserialized objects in this list. 'writer' may
   // concurrently add more objects.
   void SerializeAll(ObjectVisitor* writer);