[dart:io] Remove DART_IO_DISABLED

runtime/bin/eventhandler.cc

 // Copyright (c) 2012, 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/eventhandler.h"
 
 #include "bin/builtin.h"
 #include "bin/dartutils.h"
 #include "bin/lockers.h"
 #include "bin/socket.h"
 #include "bin/thread.h"
 
 #include "include/dart_api.h"
 
@@ skipping 123 matching lines @@
 }
 
 void FUNCTION_NAME(EventHandler_TimerMillisecondClock)(
     Dart_NativeArguments args) {
   int64_t now = TimerUtils::GetCurrentMonotonicMillis();
   Dart_SetReturnValue(args, Dart_NewInteger(now));
 }
 
 }  // namespace bin
 }  // namespace dart
-
-#endif  // !defined(DART_IO_DISABLED)