Allow simulator runs on Windows:

runtime/vm/atomic_win.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_WIN_H_
 #define VM_ATOMIC_WIN_H_
 
 #if !defined VM_ATOMIC_H_
 #error Do not include atomic_win.h directly. Use atomic.h instead.
 #endif
 
 #if !defined(TARGET_OS_WINDOWS)
 #error This file should only be included on Windows builds.
 #endif
 
 namespace dart {
 
 inline uintptr_t AtomicOperations::FetchAndIncrement(uintptr_t* p) {
-#if defined(TARGET_ARCH_X64)
+#if defined(HOST_ARCH_X64)
   return static_cast<uintptr_t>(
       InterlockedIncrement64(reinterpret_cast<LONGLONG*>(p))) - 1;
-#elif defined(TARGET_ARCH_IA32)
+#elif defined(HOST_ARCH_IA32)
   return static_cast<uintptr_t>(
       InterlockedIncrement(reinterpret_cast<LONG*>(p))) - 1;
 #else
-  UNIMPLEMENTED();
+#error Unsupported host architecture.
 #endif
 }
 
 
 inline intptr_t AtomicOperations::FetchAndAdd(intptr_t* p, intptr_t delta) {
-#if defined(TARGET_ARCH_X64)
+#if defined(HOST_ARCH_X64)
   return static_cast<intptr_t>(
       InterlockedAdd64(reinterpret_cast<LONGLONG*>(p), delta)) - delta;
-#elif defined(TARGET_ARCH_IA32)
+#elif defined(HOST_ARCH_IA32)
   return static_cast<intptr_t>(
       InterlockedAdd(reinterpret_cast<LONG*>(p), delta)) - delta;
 #else
-  UNIMPLEMENTED();
+#error Unsupported host architecture.
 #endif
 }
 
 
 #if !defined(USING_SIMULATOR)
 inline uword AtomicOperations::CompareAndSwapWord(uword* ptr,
                                                   uword old_value,
                                                   uword new_value) {
-#if defined(TARGET_ARCH_X64)
+#if defined(HOST_ARCH_X64)
   return static_cast<uword>(
       InterlockedCompareExchange64(reinterpret_cast<LONGLONG*>(ptr),
                                    static_cast<LONGLONG>(new_value),
                                    static_cast<LONGLONG>(old_value)));
-#elif defined(TARGET_ARCH_IA32)
+#elif defined(HOST_ARCH_IA32)
   return static_cast<uword>(
       InterlockedCompareExchange(reinterpret_cast<LONG*>(ptr),
                                  static_cast<LONG>(new_value),
                                  static_cast<LONG>(old_value)));
 #else
-  UNIMPLEMENTED();
+#error Unsupported host architecture.
 #endif
 }
 #endif  // !defined(USING_SIMULATOR)
 
 }  // namespace dart
 
 #endif  // VM_ATOMIC_WIN_H_