Add additional include guard in platform specific .h files

runtime/bin/eventhandler_win.h

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2012, 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_EVENTHANDLER_WIN_H_
 #define BIN_EVENTHANDLER_WIN_H_
 
+#if !defined(BIN_EVENTHANDLER_H_)
+#error Do not include eventhandler_win.h directly; use eventhandler.h instead.
+#endif
+
 #include <winsock2.h>
 #include <mswsock.h>
 
 #include "bin/builtin.h"
 
 
 // Forward declarations.
 class EventHandlerImplementation;
 class Handle;
 class FileHandle;
@@ skipping 337 matching lines @@
  private:
   ClientSocket* client_sockets_head_;
 
   int64_t timeout_;  // Time for next timeout.
   Dart_Port timeout_port_;
   HANDLE completion_port_;
 };
 
 
 #endif  // BIN_EVENTHANDLER_WIN_H_