Use patching for dart:io.

lib/io/http_session.dart

 // 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.
 
 const String _DART_SESSION_ID = "DARTSESSID";
 
 // A _HttpSession is a node in a double-linked list, with _next and _prev being
 // the previous and next pointers.
 class _HttpSession implements HttpSession {
   _HttpSession(_HttpSessionManager this._sessionManager, String this.id)
@@ skipping 136 matching lines @@
       _timer = null;
     }
   }
 
   Map<String, _HttpSession> _sessions;
   int _sessionTimeout = 20 * 60;  // 20 mins.
   _HttpSession _head;
   _HttpSession _tail;
   Timer _timer;
 
-  static Uint8List _getRandomBytes(int count)
-      native "Crypto_GetRandomBytes";
+  external static Uint8List _getRandomBytes(int count);
 }