Really remove io support when dart:io is unsupported.

runtime/bin/file_system_watcher_linux.cc

Index: runtime/bin/file_system_watcher_linux.cc
diff --git a/runtime/bin/file_system_watcher_linux.cc b/runtime/bin/file_system_watcher_linux.cc
index e692f0d3a8a18394ceca1d15eee5c2d2e43858cc..ef4eaefb757e7e8d98e2b0324333957970a6499e 100644
--- a/runtime/bin/file_system_watcher_linux.cc
+++ b/runtime/bin/file_system_watcher_linux.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_LINUX)
 
@@ -145,3 +147,5 @@ Dart_Handle FileSystemWatcher::ReadEvents(intptr_t id, intptr_t path_id) {
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_LINUX)
+
+#endif  // !defined(DART_IO_DISABLED)