Really remove io support when dart:io is unsupported.

runtime/bin/eventhandler_macos.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 "platform/globals.h"
 #if defined(TARGET_OS_MACOS)
 
 #include "bin/eventhandler.h"
 #include "bin/eventhandler_macos.h"
 
 #include <errno.h>  // NOLINT
 #include <fcntl.h>  // NOLINT
 #include <pthread.h>  // NOLINT
 #include <stdio.h>  // NOLINT
@@ skipping 487 matching lines @@
 
 uint32_t EventHandlerImplementation::GetHashmapHashFromFd(intptr_t fd) {
   // The hashmap does not support keys with value 0.
   return dart::Utils::WordHash(fd + 1);
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_MACOS)
+
+#endif  // !defined(DART_IO_DISABLED)