Really remove io support when dart:io is unsupported.

runtime/bin/socket_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(TARGET_OS_LINUX)
 
 #include "bin/socket.h"
 #include "bin/socket_linux.h"
 
 #include <errno.h>  // NOLINT
 #include <ifaddrs.h>  // NOLINT
 #include <net/if.h>  // NOLINT
 #include <netinet/tcp.h>  // NOLINT
@@ skipping 563 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(TARGET_OS_LINUX)
+
+#endif  // !defined(DART_IO_DISABLED)