Use #if TARGET_OS_IOS instead of #ifdef TARGET_OS_IOS.

runtime/bin/process_macos.cc

 // Copyright (c) 2012, 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_MACOS)
 
 #include "bin/process.h"
 
-#if !defined(TARGET_OS_IOS)
+#if !TARGET_OS_IOS
 #include <crt_externs.h>  // NOLINT
 #endif
 #include <errno.h>  // NOLINT
 #include <fcntl.h>  // NOLINT
 #include <poll.h>  // NOLINT
 #include <signal.h>  // NOLINT
 #include <stdio.h>  // NOLINT
 #include <stdlib.h>  // NOLINT
 #include <string.h>  // NOLINT
 #include <unistd.h>  // NOLINT
@@ skipping 423 matching lines @@
 
     if (TEMP_FAILURE_RETRY(dup2(read_err_[1], STDERR_FILENO)) == -1) {
       ReportChildError();
     }
 
     if (working_directory_ != NULL &&
         TEMP_FAILURE_RETRY(chdir(working_directory_)) == -1) {
       ReportChildError();
     }
 
-#if !defined(TARGET_OS_IOS)
+#if !TARGET_OS_IOS
     if (program_environment_ != NULL) {
       // On MacOS you have to do a bit of magic to get to the
       // environment strings.
       char*** environ = _NSGetEnviron();
       *environ = program_environment_;
     }
 #endif
 
     VOID_TEMP_FAILURE_RETRY(
         execvp(path_, const_cast<char* const*>(program_arguments_)));
@@ skipping 597 matching lines @@
     bzero(&act, sizeof(act));
     act.sa_handler = SIG_DFL;
     VOID_NO_RETRY_EXPECTED(sigaction(signal, &act, NULL));
   }
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_MACOS)