Don't overload FreeListElement for become.

runtime/vm/verifier.cc

Index: runtime/vm/verifier.cc
diff --git a/runtime/vm/verifier.cc b/runtime/vm/verifier.cc
index 78b4697712dad403e95efdeeac432ec31db91344..5491f1dbdfef32d3002e24d9236a7dd1bd0045cc 100644
--- a/runtime/vm/verifier.cc
+++ b/runtime/vm/verifier.cc
@@ -20,7 +20,7 @@ namespace dart {
 void VerifyObjectVisitor::VisitObject(RawObject* raw_obj) {
   if (raw_obj->IsHeapObject()) {
     uword raw_addr = RawObject::ToAddr(raw_obj);
-    if (raw_obj->IsFreeListElement()) {
+    if (raw_obj->IsFreeListElement() || raw_obj->IsForwardingCorpse()) {
       if (raw_obj->IsMarked()) {
         FATAL1("Marked free list element encountered %#" Px "\n", raw_addr);
       }