Ensure proper response to chrome client_max_window_bits

sdk/lib/io/websocket_impl.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 _webSocketGUID = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11";
 const String _clientNoContextTakeover = "client_no_context_takeover";
 const String _serverNoContextTakeover = "server_no_context_takeover";
 const String _clientMaxWindowBits = "client_max_window_bits";
@@ skipping 440 matching lines @@
       }).then(upgrade);
     } else {
       return upgrade(null);
     }
   }
 
   static _WebSocketPerMessageDeflate _negotiateCompression(HttpRequest request,
       HttpResponse response, CompressionOptions compression) {
     var extensionHeader = request.headers.value("Sec-WebSocket-Extensions");
 
-    if (extensionHeader == null) {
-      extensionHeader = "";
-    }
+    extensionHeader ??= "";
 
     var hv = HeaderValue.parse(extensionHeader, valueSeparator: ',');
     if (compression.enabled && hv.value == _WebSocketImpl.PER_MESSAGE_DEFLATE) {
       var info = compression._createHeader(hv);
 
       response.headers.add("Sec-WebSocket-Extensions", info[0]);
       var serverNoContextTakeover =
           hv.parameters.containsKey(_serverNoContextTakeover);
       var clientNoContextTakeover =
           hv.parameters.containsKey(_clientNoContextTakeover);
@@ skipping 757 matching lines @@
     return code != null &&
         (code < WebSocketStatus.NORMAL_CLOSURE ||
             code == WebSocketStatus.RESERVED_1004 ||
             code == WebSocketStatus.NO_STATUS_RECEIVED ||
             code == WebSocketStatus.ABNORMAL_CLOSURE ||
             (code > WebSocketStatus.INTERNAL_SERVER_ERROR &&
                 code < WebSocketStatus.RESERVED_1015) ||
             (code >= WebSocketStatus.RESERVED_1015 && code < 3000));
   }
 }