Don't use linked maps/sets in http internals.

sdk/lib/io/http_session.dart

 // 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.
 
 part of dart.io;
 
 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 {
   // Destroyed marked. Used by the http connection to see if a session is valid.
   bool _destroyed = false;
   bool _isNew = true;
   DateTime _lastSeen;
   Function _timeoutCallback;
   _HttpSessionManager _sessionManager;
   // Pointers in timeout queue.
   _HttpSession _prev;
   _HttpSession _next;
 
-  final Map _data = new Map();
+  final Map _data = new HashMap();
 
   _HttpSession(_HttpSessionManager this._sessionManager, String this.id)
     : _lastSeen = new DateTime.now();
 
   void destroy() {
     _destroyed = true;
     _sessionManager._removeFromTimeoutQueue(this);
     _sessionManager._sessions.remove(id);
   }
 
@@ skipping 136 matching lines @@
     }
   }
 
   Map<String, _HttpSession> _sessions;
   int _sessionTimeout = 20 * 60;  // 20 mins.
   _HttpSession _head;
   _HttpSession _tail;
   Timer _timer;
 }