[vm] Allocate old in object-graph-copy's slow path.

If we have reached the slow path, the message's subgraph is very likely to be large.

TEST=ci
Change-Id: Ie8a18c78936ae8a53f30dd61da3650e684c09dfa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/249081
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
diff --git a/runtime/vm/object_graph_copy.cc b/runtime/vm/object_graph_copy.cc
index 03f24ba..f2cf88e 100644
--- a/runtime/vm/object_graph_copy.cc
+++ b/runtime/vm/object_graph_copy.cc
@@ -259,7 +259,7 @@
 #else
   const bool compressed = false;
 #endif
-  return Object::Allocate(cid, size, Heap::kNew, compressed);
+  return Object::Allocate(cid, size, Heap::kOld, compressed);
 }
 
 DART_FORCE_INLINE