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

This reverts commit d0a60d67c9820d0a146f48adf0c1111b87e840f8.

Reason for revert: Slow path transition seems to occur earlier than I expected

Original change's description:
> [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>

TBR=kustermann@google.com,rmacnak@google.com

Change-Id: I61faddd37e424a742427ae7c270cdb8b0a13c418
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/255300
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>
diff --git a/runtime/vm/object_graph_copy.cc b/runtime/vm/object_graph_copy.cc
index 37aa730..991f1a2 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::kOld, compressed);
+  return Object::Allocate(cid, size, Heap::kNew, compressed);
 }
 
 DART_FORCE_INLINE