Compute next field offset correctly.

runtime/vm/freelist.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_FREELIST_H_
 #define VM_FREELIST_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/bit_set.h"
@@ skipping 25 matching lines @@
   static FreeListElement* AsElement(uword addr, intptr_t size);
 
   static void InitOnce();
 
   // Used to allocate class for free list elements in Object::InitOnce.
   class FakeInstance {
    public:
     FakeInstance() { }
     static cpp_vtable vtable() { return 0; }
     static intptr_t InstanceSize() { return 0; }
+    static intptr_t NextFieldOffset() { return -kWordSize; }
     static const ClassId kClassId = kFreeListElement;
     static bool IsInstance() { return true; }
 
    private:
     DISALLOW_ALLOCATION();
     DISALLOW_COPY_AND_ASSIGN(FakeInstance);
   };
 
  private:
   // This layout mirrors the layout of RawObject.
@@ skipping 43 matching lines @@
   BitSet<kNumLists> free_map_;
 
   FreeListElement* free_lists_[kNumLists + 1];
 
   DISALLOW_COPY_AND_ASSIGN(FreeList);
 };
 
 }  // namespace dart
 
 #endif  // VM_FREELIST_H_