The psuedo-objects are not instances: give them cids above kInstanceCid.

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 132 matching lines @@
 #define DEFINE_FORWARD_DECLARATION(clazz)                                      \
   class Raw##clazz;
 CLASS_LIST(DEFINE_FORWARD_DECLARATION)
 #undef DEFINE_FORWARD_DECLARATION
 
 
 enum ClassId {
   // Illegal class id.
   kIllegalCid = 0,
 
+  // 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,
+
   // List of Ids for predefined classes.
 #define DEFINE_OBJECT_KIND(clazz)                                              \
   k##clazz##Cid,
 CLASS_LIST(DEFINE_OBJECT_KIND)
 #undef DEFINE_OBJECT_KIND
 
 #define DEFINE_OBJECT_KIND(clazz)                                              \
   kTypedData##clazz##Cid,
 CLASS_LIST_TYPED_DATA(DEFINE_OBJECT_KIND)
 #undef DEFINE_OBJECT_KIND
@@ skipping 11 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 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,
   kObjectAlignmentLog2 = kWordSizeLog2 + 1,
@@ skipping 2253 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_