[dart:io] Remove DART_IO_DISABLED

runtime/bin/socket_base_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(HOST_OS_FUCHSIA)
 
 #include "bin/socket_base.h"
 
 // TODO(MG-766): If/when Fuchsia adds getifaddrs(), use that instead of the
 // ioctl in netconfig.h.
 #include <apps/netstack/apps/include/netconfig.h>
 #include <errno.h>        // NOLINT
 #include <fcntl.h>        // NOLINT
@@ skipping 382 matching lines @@
                                 int interfaceIndex) {
   LOG_ERR("SocketBase::LeaveMulticast is unimplemented\n");
   UNIMPLEMENTED();
   return false;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(HOST_OS_FUCHSIA)
-
-#endif  // !defined(DART_IO_DISABLED)