Don't overload FreeListElement for become.

runtime/vm/verifier.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/verifier.h"
 
 #include "platform/assert.h"
 #include "vm/dart.h"
 #include "vm/dart_api_state.h"
 #include "vm/freelist.h"
 #include "vm/heap.h"
 #include "vm/isolate.h"
 #include "vm/object.h"
 #include "vm/object_set.h"
 #include "vm/raw_object.h"
 #include "vm/stack_frame.h"
 
 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);
       }
     } else {
       switch (mark_expectation_) {
        case kForbidMarked:
         if (raw_obj->IsMarked()) {
           FATAL1("Marked object encountered %#" Px "\n", raw_addr);
         }
         break;
@@ skipping 43 matching lines @@
   // Visit all strongly reachable objects.
   isolate->IterateObjectPointers(&visitor,
                                  StackFrameIterator::kValidateFrames);
   VerifyWeakPointersVisitor weak_visitor(&visitor);
   // Visit weak handles and prologue weak handles.
   isolate->VisitWeakPersistentHandles(&weak_visitor);
   delete allocated_set;
 }
 
 }  // namespace dart