[dart:io] Remove DART_IO_DISABLED

runtime/bin/socket_base_win.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_WINDOWS)
 
 #include "bin/socket_base.h"
 
 #include "bin/builtin.h"
 #include "bin/eventhandler.h"
 #include "bin/file.h"
 #include "bin/lockers.h"
 #include "bin/log.h"
@@ skipping 404 matching lines @@
   mreq.gr_interface = interfaceIndex;
   memmove(&mreq.gr_group, &addr.ss, SocketAddress::GetAddrLength(addr));
   return setsockopt(handle->socket(), proto, MCAST_LEAVE_GROUP,
                     reinterpret_cast<char*>(&mreq), sizeof(mreq)) == 0;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(HOST_OS_WINDOWS)
-
-#endif  // !defined(DART_IO_DISABLED)