[dart:io] Remove DART_IO_DISABLED

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.
 
-#if !defined(DART_IO_DISABLED)
-
 #include "platform/globals.h"
 #if defined(HOST_OS_MACOS)
 
 #include "bin/process.h"
 
 #if !HOST_OS_IOS
 #include <crt_externs.h>  // NOLINT
 #endif
 #include <errno.h>      // NOLINT
 #include <fcntl.h>      // NOLINT
@@ skipping 1058 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(HOST_OS_MACOS)
-
-#endif  // !defined(DART_IO_DISABLED)