Allow simulator runs on Windows:

runtime/vm/atomic_win.h

Index: runtime/vm/atomic_win.h
diff --git a/runtime/vm/atomic_win.h b/runtime/vm/atomic_win.h
index 433d32e86d52db439b6ea964e0e1bd2e300ba171..0c31e592aa6b4d1bee4cb70f00f0df0c2562b8fb 100644
--- a/runtime/vm/atomic_win.h
+++ b/runtime/vm/atomic_win.h
@@ -16,27 +16,27 @@
 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
 }
 
@@ -45,18 +45,18 @@ inline intptr_t AtomicOperations::FetchAndAdd(intptr_t* p, intptr_t delta) {
 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)