Don't overload FreeListElement for become.

runtime/vm/object_test.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 "platform/globals.h"
 
 #include "vm/assembler.h"
 #include "vm/class_finalizer.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_entry.h"
@@ skipping 4108 matching lines @@
 #ifndef PRODUCT
 
 
 class ObjectAccumulator : public ObjectVisitor {
  public:
   explicit ObjectAccumulator(GrowableArray<Object*>* objects)
       : objects_(objects) { }
   virtual ~ObjectAccumulator() { }
   virtual void VisitObject(RawObject* obj) {
     // Free-list elements cannot even be wrapped in handles.
-    if (obj->IsFreeListElement()) {
+    if (obj->IsFreeListElement() || obj->IsForwardingCorpse()) {
       return;
     }
     Object& handle = Object::Handle(obj);
     // Skip some common simple objects to run in reasonable time.
     if (handle.IsString() ||
         handle.IsArray() ||
         handle.IsLiteralToken()) {
       return;
     }
     objects_->Add(&handle);
@@ skipping 578 matching lines @@
   String& test = String::Handle();
   String& result = String::Handle();
   for (size_t i = 0; i < ARRAY_SIZE(tests); i++) {
     test = String::New(tests[i].in);
     result = String::ScrubName(test);
     EXPECT_STREQ(tests[i].out, result.ToCString());
   }
 }
 
 }  // namespace dart