Really remove io support when dart:io is unsupported.

runtime/bin/file_system_watcher_macos.cc

Index: runtime/bin/file_system_watcher_macos.cc
diff --git a/runtime/bin/file_system_watcher_macos.cc b/runtime/bin/file_system_watcher_macos.cc
index ff6428849ecab1844225f4a5b8eb8a09ebfafcae..7bc0e0c819e94fbbe7184d0ad3d6bd56339c1b07 100644
--- a/runtime/bin/file_system_watcher_macos.cc
+++ b/runtime/bin/file_system_watcher_macos.cc
@@ -2,6 +2,8 @@
 // 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)
 
@@ -458,3 +460,5 @@ intptr_t FileSystemWatcher::WatchPath(intptr_t id,
 
 #endif  // !TARGET_OS_IOS
 #endif  // defined(TARGET_OS_MACOS)
+
+#endif  // !defined(DART_IO_DISABLED)