[dart:io] Remove DART_IO_DISABLED

runtime/bin/socket.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 "bin/socket.h"
 
 #include "bin/dartutils.h"
 #include "bin/eventhandler.h"
 #include "bin/io_buffer.h"
 #include "bin/isolate_data.h"
 #include "bin/lockers.h"
 #include "bin/thread.h"
 #include "bin/utils.h"
 
@@ skipping 943 matching lines @@
       Dart_GetNativeInstanceField(socket_obj, kSocketIdNativeField, &id);
   if (Dart_IsError(err)) {
     Dart_PropagateError(err);
   }
   Socket* socket = reinterpret_cast<Socket*>(id);
   return socket;
 }
 
 }  // namespace bin
 }  // namespace dart
-
-#endif  // !defined(DART_IO_DISABLED)