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

runtime/vm/atomic_macos.h

Index: runtime/vm/atomic_macos.h
diff --git a/runtime/vm/atomic_macos.h b/runtime/vm/atomic_macos.h
index b56d2a0abd7bc5ea5e0c6dbaeeb2b345710ce0c2..31d219b027457db06b6d6687a1850c58d07d27a8 100644
--- a/runtime/vm/atomic_macos.h
+++ b/runtime/vm/atomic_macos.h
@@ -32,6 +32,13 @@ inline uword AtomicOperations::CompareAndSwapWord(uword* ptr,
                                                   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