Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/file_system_watcher_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.
 
 #if !defined(DART_IO_DISABLED)
 
 #include "platform/globals.h"
-#if defined(TARGET_OS_FUCHSIA)
+#if defined(HOST_OS_FUCHSIA)
 
 #include "bin/file_system_watcher.h"
 
 namespace dart {
 namespace bin {
 
 Dart_Handle FileSystemWatcher::ReadEvents(intptr_t id, intptr_t path_id) {
   UNIMPLEMENTED();
   return DartUtils::NewDartOSError();
 }
@@ skipping 29 matching lines @@
                                       const char* path,
                                       int events,
                                       bool recursive) {
   UNIMPLEMENTED();
   return -1;
 }
 
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_FUCHSIA)
+#endif  // defined(HOST_OS_FUCHSIA)
 
 #endif  // !defined(DART_IO_DISABLED)