Add a CompareAndSwapUint32 variant to AtomicOperations so that it is possible to a CAS of a 32 bit …

runtime/vm/atomic_macos.h

 // Copyright (c) 2013, 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_ATOMIC_MACOS_H_
 #define VM_ATOMIC_MACOS_H_
 
 #if !defined VM_ATOMIC_H_
 #error Do not include atomic_macos.h directly. Use atomic.h instead.
 #endif
@@ skipping 14 matching lines @@
   return __sync_fetch_and_sub(p, 1);
 }
 
 
 #if !defined(USING_SIMULATOR)
 inline uword AtomicOperations::CompareAndSwapWord(uword* ptr,
                                                   uword old_value,
                                                   uword new_value) {
   return __sync_val_compare_and_swap(ptr, old_value, new_value);
 }
+
+
+inline uint32_t AtomicOperations::CompareAndSwapUint32(uint32_t* ptr,
+                                                       uint32_t old_value,
+                                                       uint32_t new_value) {
+  return __sync_val_compare_and_swap(ptr, old_value, new_value);
+}
 #endif  // !defined(USING_SIMULATOR)
 
 }  // namespace dart
 
 #endif  // VM_ATOMIC_MACOS_H_