Really remove io support when dart:io is unsupported.

runtime/bin/file_system_watcher.cc

Index: runtime/bin/file_system_watcher.cc
diff --git a/runtime/bin/file_system_watcher.cc b/runtime/bin/file_system_watcher.cc
index 5d8c0b8eaca853bd1b72b45d88b78637dcc84c73..317f59f2d76ca786397c777992d04fa581c35a83 100644
--- a/runtime/bin/file_system_watcher.cc
+++ b/runtime/bin/file_system_watcher.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 "bin/file_system_watcher.h"
 
 #include "bin/builtin.h"
@@ -73,3 +75,5 @@ void FUNCTION_NAME(FileSystemWatcher_GetSocketId)(Dart_NativeArguments args) {
 
 }  // namespace bin
 }  // namespace dart
+
+#endif  // !defined(DART_IO_DISABLED)