Don't overload FreeListElement for become.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 74cf17600a7960dfc68869a8e14d5e4189738a9e..518b0274d127508404f81076410d6d8c555ee358 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -181,9 +181,12 @@ CLASS_LIST_TYPED_DATA(DEFINE_OBJECT_KIND)
   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,
 };
@@ -442,6 +445,12 @@ CLASS_LIST_TYPED_DATA(DEFINE_IS_CID)
   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_;