Revert "[vm] Implement more efficient CAS in simarm/simarm64 modes"

runtime/vm/atomic_fuchsia.h

 // Copyright (c) 2016, 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 RUNTIME_VM_ATOMIC_FUCHSIA_H_
 #define RUNTIME_VM_ATOMIC_FUCHSIA_H_
 
 #if !defined RUNTIME_VM_ATOMIC_H_
 #error Do not include atomic_fuchsia.h directly. Use atomic.h instead.
 #endif
@@ skipping 25 matching lines @@
 }
 
 inline intptr_t AtomicOperations::FetchAndDecrement(intptr_t* p) {
   return __sync_fetch_and_sub(p, 1);
 }
 
 inline void AtomicOperations::DecrementBy(intptr_t* p, intptr_t value) {
   __sync_fetch_and_sub(p, value);
 }
 
+#if !defined(USING_SIMULATOR_ATOMICS)
 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_ATOMICS)
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_ATOMIC_FUCHSIA_H_