Don't overload FreeListElement for become.

runtime/vm/raw_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_RAW_OBJECT_H_
 #define VM_RAW_OBJECT_H_
 
 #include "platform/assert.h"
 #include "vm/atomic.h"
 #include "vm/globals.h"
@@ skipping 163 matching lines @@
 #undef DEFINE_OBJECT_KIND
 
   kByteBufferCid,
 
   // The following entries do not describe a predefined class, but instead
   // are class indexes for pre-allocated instance (Null, dynamic and Void).
   kNullCid,
   kDynamicCid,
   kVoidCid,
 
-  // The following entry does not describe a real class, but instead it is an
-  // id which is used to identify free list elements in the heap.
+  // The following entries describes classes for pseudo-objects in the heap
+  // that should never be reachable from live objects. Free list elements
+  // maintain the free list for old space, and forwarding corpses are used to
+  // implement one-way become.
   kFreeListElement,
+  kForwardingCorpse,
 
   kNumPredefinedCids,
 };
 
 enum ObjectAlignment {
   // Alignment offsets are used to determine object age.
   kNewObjectAlignmentOffset = kWordSize,
   kOldObjectAlignmentOffset = 0,
   // Object sizes are aligned to kObjectAlignment.
   kObjectAlignment = 2 * kWordSize,
@@ skipping 238 matching lines @@
 
   bool IsStringInstance() const {
     return IsStringClassId(GetClassId());
   }
   bool IsDartInstance() const {
     return (!IsHeapObject() || (GetClassId() >= kInstanceCid));
   }
   bool IsFreeListElement() const {
     return ((GetClassId() == kFreeListElement));
   }
+  bool IsForwardingCorpse() const {
+    return ((GetClassId() == kForwardingCorpse));
+  }
+  bool IsPseudoObject() const {
+    return IsFreeListElement() || IsForwardingCorpse();
+  }
 
   intptr_t Size() const {
     uword tags = ptr()->tags_;
     intptr_t result = SizeTag::decode(tags);
     if (result != 0) {
       ASSERT(result == SizeFromClass());
       return result;
     }
     result = SizeFromClass();
     ASSERT(result > SizeTag::kMaxSizeTag);
@@ skipping 1965 matching lines @@
   COMPILE_ASSERT(kExternalTypedDataInt8ArrayCid ==
                  kTypedDataInt8ArrayViewCid + 15);
   COMPILE_ASSERT(kByteBufferCid == kExternalTypedDataInt8ArrayCid + 14);
   COMPILE_ASSERT(kNullCid == kByteBufferCid + 1);
   return (kNullCid - kTypedDataInt8ArrayCid);
 }
 
 }  // namespace dart
 
 #endif  // VM_RAW_OBJECT_H_