[dart:io] Remove DART_IO_DISABLED

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 966436146015b5dcc72e17d8b4b29ee466300e02..fc7666540041bfdc36da557e77ed7f7f67d1c0aa 100644
--- a/runtime/bin/file_system_watcher.cc
+++ b/runtime/bin/file_system_watcher.cc
@@ -2,8 +2,6 @@
 // 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"
@@ -69,5 +67,3 @@ void FUNCTION_NAME(FileSystemWatcher_GetSocketId)(Dart_NativeArguments args) {
 
 }  // namespace bin
 }  // namespace dart
-
-#endif  // !defined(DART_IO_DISABLED)