[dart:io] Remove DART_IO_DISABLED

runtime/bin/file_system_watcher.h

 // 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.
 
 #ifndef RUNTIME_BIN_FILE_SYSTEM_WATCHER_H_
 #define RUNTIME_BIN_FILE_SYSTEM_WATCHER_H_
 
-#if defined(DART_IO_DISABLED)
-#error "file_system_watcher.h can only be included on builds with IO enabled"
-#endif
-
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/types.h>
 
 #include "bin/builtin.h"
 #include "bin/dartutils.h"
 
 namespace dart {
 namespace bin {
@@ skipping 29 matching lines @@
   static Dart_Handle ReadEvents(intptr_t id, intptr_t path_id);
 
  private:
   DISALLOW_COPY_AND_ASSIGN(FileSystemWatcher);
 };
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // RUNTIME_BIN_FILE_SYSTEM_WATCHER_H_