Make file system watcher compile on Mac OS 106, and add a runtime-call to test if the system suppor…

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 BIN_FILE_SYSTEM_WATCHER_H_
 #define BIN_FILE_SYSTEM_WATCHER_H_
 
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
@@ skipping 16 matching lines @@
     kModefyAttribute = 1 << 4
   };
 
   struct Event {
     intptr_t path_id;
     int event;
     const char* filename;
     int link;
   };
 
+  static bool IsSupported();
   static intptr_t WatchPath(const char* path, int events, bool recursive);
   static void UnwatchPath(intptr_t id);
   static intptr_t GetSocketId(intptr_t id);
   static Dart_Handle ReadEvents(intptr_t id);
 
  private:
   DISALLOW_COPY_AND_ASSIGN(FileSystemWatcher);
 };
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // BIN_FILE_SYSTEM_WATCHER_H_