Don't set serverHeader by default.

sdk/lib/io/http_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 int _HEADERS_BUFFER_SIZE = 8 * 1024;
 
 class _HttpIncoming extends Stream<List<int>> {
   final int _transferLength;
@@ skipping 1989 matching lines @@
 
   bool get _isActive => _state == _ACTIVE;
   bool get _isIdle => _state == _IDLE;
   bool get _isClosing => _state == _CLOSING;
   bool get _isDetached => _state == _DETACHED;
 }
 
 
 // HTTP server waiting for socket connections.
 class _HttpServer extends Stream<HttpRequest> implements HttpServer {
-  String serverHeader = _getHttpVersion();
+  String serverHeader;
 
   Duration idleTimeout = const Duration(seconds: 120);
 
   static Future<HttpServer> bind(address, int port, int backlog) {
     return ServerSocket.bind(address, port, backlog: backlog).then((socket) {
       return new _HttpServer._(socket, true);
     });
   }
 
   static Future<HttpServer> bindSecure(address,
@@ skipping 547 matching lines @@
   const _RedirectInfo(this.statusCode, this.method, this.location);
 }
 
 String _getHttpVersion() {
   var version = Platform.version;
   // Only include major and minor version numbers.
   int index = version.indexOf('.', version.indexOf('.') + 1);
   version = version.substring(0, index);
   return 'Dart/$version (dart:io)';
 }