Really remove io support when dart:io is unsupported.

runtime/bin/file_system_watcher_macos.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_MACOS)
 
 #include "bin/file_system_watcher.h"
 
 #if !TARGET_OS_IOS
 
 #include <errno.h>  // NOLINT
 #include <fcntl.h>  // NOLINT
 #include <unistd.h>  // NOLINT
@@ skipping 436 matching lines @@
                                       int events,
                                       bool recursive) {
   return -1;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // !TARGET_OS_IOS
 #endif  // defined(TARGET_OS_MACOS)
+
+#endif  // !defined(DART_IO_DISABLED)