Don't overload FreeListElement for become.

runtime/vm/object.h

 // 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.
 
 #ifndef VM_OBJECT_H_
 #define VM_OBJECT_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/utils.h"
@@ skipping 8501 matching lines @@
 DART_FORCE_INLINE void Object::SetRaw(RawObject* value) {
   // NOTE: The assignment "raw_ = value" should be the first statement in
   // this function. Also do not use 'value' in this function after the
   // assignment (use 'raw_' instead).
   raw_ = value;
   if ((reinterpret_cast<uword>(value) & kSmiTagMask) == kSmiTag) {
     set_vtable(Smi::handle_vtable_);
     return;
   }
   intptr_t cid = value->GetClassId();
   // Free-list elements cannot be wrapped in a handle.

[siva, 2016/06/02 00:23:36]

Update comment (Free List and Forwarding corpse) elements

   ASSERT(cid != kFreeListElement);
+  ASSERT(cid != kForwardingCorpse);
   if (cid >= kNumPredefinedCids) {
     cid = kInstanceCid;
   }
   set_vtable(builtin_vtables_[cid]);
 #if defined(DEBUG)
   if (FLAG_verify_handles) {
     Isolate* isolate = Isolate::Current();
     Heap* isolate_heap = isolate->heap();
     Heap* vm_isolate_heap = Dart::vm_isolate()->heap();
     ASSERT(isolate_heap->Contains(RawObject::ToAddr(raw_)) ||
@@ skipping 177 matching lines @@
 
 inline void TypeArguments::SetHash(intptr_t value) const {
   // This is only safe because we create a new Smi, which does not cause
   // heap allocation.
   StoreSmi(&raw_ptr()->hash_, Smi::New(value));
 }
 
 }  // namespace dart
 
 #endif  // VM_OBJECT_H_