Really remove io support when dart:io is unsupported.

runtime/bin/file_system_watcher.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 "bin/file_system_watcher.h"
 
 #include "bin/builtin.h"
 #include "bin/dartutils.h"
 #include "bin/utils.h"
 
 #include "include/dart_api.h"
 
 namespace dart {
 namespace bin {
@@ skipping 51 matching lines @@
 
 void FUNCTION_NAME(FileSystemWatcher_GetSocketId)(Dart_NativeArguments args) {
   intptr_t id = DartUtils::GetIntptrValue(Dart_GetNativeArgument(args, 0));
   intptr_t path_id = DartUtils::GetIntptrValue(Dart_GetNativeArgument(args, 1));
   int socket_id = FileSystemWatcher::GetSocketId(id, path_id);
   Dart_SetReturnValue(args, Dart_NewInteger(socket_id));
 }
 
 }  // namespace bin
 }  // namespace dart
+
+#endif  // !defined(DART_IO_DISABLED)