Don't create lower-case copies of already lower-case header names, in http parser.

sdk/lib/io/http_parser.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;
 
 // Global constants.
 class _Const {
   // Bytes for "HTTP".
   static const HTTP = const [72, 84, 84, 80];
@@ skipping 505 matching lines @@
             if (headerField == "transfer-encoding" &&
                        headerValue.toLowerCase() == "chunked") {
               _chunked = true;
             }
             if (headerField == "connection") {
               List<String> tokens = _tokenizeFieldValue(headerValue);
               for (int i = 0; i < tokens.length; i++) {
                 if (tokens[i].toLowerCase() == "upgrade") {
                   _connectionUpgrade = true;
                 }
-                _headers.add(headerField, tokens[i]);
+                _headers._add(headerField, tokens[i]);
               }
             } else {
-              _headers.add(headerField, headerValue);
+              _headers._add(headerField, headerValue);
             }
             _headerField.clear();
             _headerValue.clear();
 
             if (byte == _CharCode.CR) {
               _state = _State.HEADER_ENDING;
             } else {
               // Start of new header field.
               _headerField.add(_toLowerCase(byte));
               _state = _State.HEADER_FIELD;
@@ skipping 443 matching lines @@
   _HttpHeaders _headers;
 
   // The current incoming connection.
   _HttpIncoming _incoming;
   StreamSubscription _socketSubscription;
   bool _paused = true;
   bool _bodyPaused = false;
   StreamController<_HttpIncoming> _controller;
   StreamController<List<int>> _bodyController;
 }