[dart:io] Remove DART_IO_DISABLED

runtime/bin/eventhandler_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/eventhandler.h"
 #include "bin/eventhandler_win.h"
 
 #include <fcntl.h>     // NOLINT
 #include <io.h>        // NOLINT
 #include <mswsock.h>   // NOLINT
 #include <winsock2.h>  // NOLINT
@@ skipping 1465 matching lines @@
 }
 
 void EventHandlerImplementation::Shutdown() {
   SendData(kShutdownId, 0, 0);
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(HOST_OS_WINDOWS)
-
-#endif  // !defined(DART_IO_DISABLED)