Change signature of atomic increment/decrement functions to return void.

runtime/vm/atomic_linux.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_LINUX_H_
 #define VM_ATOMIC_LINUX_H_
 
 #if !defined VM_ATOMIC_H_
 #error Do not include atomic_linux.h directly. Use atomic.h instead.
 #endif
 
 #if !defined(TARGET_OS_LINUX)
 #error This file should only be included on Linux builds.
 #endif
 
 namespace dart {
 
 
 inline uintptr_t AtomicOperations::FetchAndIncrement(uintptr_t* p) {
   return __sync_fetch_and_add(p, 1);
 }
 
 
-inline uintptr_t AtomicOperations::FetchAndIncrementBy(intptr_t* p,
-                                                       intptr_t value) {
-  return __sync_fetch_and_add(p, value);
+inline void AtomicOperations::IncrementBy(intptr_t* p, intptr_t value) {
+  __sync_fetch_and_add(p, value);
 }
 
 
 inline uintptr_t AtomicOperations::FetchAndDecrement(uintptr_t* p) {
   return __sync_fetch_and_sub(p, 1);
 }
 
 
-inline uintptr_t AtomicOperations::FetchAndDecrementBy(intptr_t* p,
-                                                       intptr_t value) {
-  return __sync_fetch_and_sub(p, value);
+inline void AtomicOperations::DecrementBy(intptr_t* p, intptr_t value) {
+  __sync_fetch_and_sub(p, value);
 }
 
 
 #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_LINUX_H_