Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/utils_fuchsia.cc

 // 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.
 
 #include "platform/globals.h"
-#if defined(TARGET_OS_FUCHSIA)
+#if defined(HOST_OS_FUCHSIA)
 
 #include <errno.h>
 #include <magenta/syscalls.h>
 #include <magenta/types.h>
 
 #include "bin/utils.h"
 #include "platform/assert.h"
 #include "platform/utils.h"
 
 namespace dart {
@@ skipping 80 matching lines @@
 
 
 void TimerUtils::Sleep(int64_t millis) {
   mx_nanosleep(millis * kMicrosecondsPerMillisecond *
                kNanosecondsPerMicrosecond);
 }
 
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_FUCHSIA)
+#endif  // defined(HOST_OS_FUCHSIA)