Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/socket_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/socket.h"
 #include "bin/socket_fuchsia.h"
 
 #include <errno.h>        // NOLINT
 #include <fcntl.h>        // NOLINT
 #include <ifaddrs.h>      // NOLINT
 #include <net/if.h>       // NOLINT
 #include <netinet/tcp.h>  // NOLINT
 #include <stdio.h>        // NOLINT
@@ skipping 508 matching lines @@
                             const RawAddr&,
                             int interfaceIndex) {
   LOG_ERR("Socket::LeaveMulticast is unimplemented\n");
   UNIMPLEMENTED();
   return false;
 }
 
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_FUCHSIA)
+#endif  // defined(HOST_OS_FUCHSIA)
 
 #endif  // !defined(DART_IO_DISABLED)