[dart:io] Remove DART_IO_DISABLED

runtime/bin/socket_base_linux.cc

 // Copyright (c) 2013, 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_LINUX)
 
 #include "bin/socket_base.h"
 
 #include <errno.h>        // NOLINT
 #include <ifaddrs.h>      // NOLINT
 #include <net/if.h>       // NOLINT
 #include <netinet/tcp.h>  // NOLINT
 #include <stdio.h>        // NOLINT
@@ skipping 397 matching lines @@
   mreq.gr_interface = interfaceIndex;
   memmove(&mreq.gr_group, &addr.ss, SocketAddress::GetAddrLength(addr));
   return NO_RETRY_EXPECTED(setsockopt(fd, proto, MCAST_LEAVE_GROUP, &mreq,
                                       sizeof(mreq))) == 0;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(HOST_OS_LINUX)
-
-#endif  // !defined(DART_IO_DISABLED)