Rename TARGET_OS_* to HOST_OS_*.

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
 
-#if !defined(TARGET_OS_FUCHSIA)
+#if !defined(HOST_OS_FUCHSIA)
 #error This file should only be included on Fuchsia builds.
 #endif
 
 namespace dart {
 
 inline uintptr_t AtomicOperations::FetchAndIncrement(uintptr_t* p) {
   return __sync_fetch_and_add(p, 1);
 }
 
 
@@ skipping 38 matching lines @@
 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_