Throw exception when trying to modify HTTP status code when header is already sent

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;
 
 class _HttpIncoming extends Stream<List<int>> {
   final int _transferLength;
   final Completer _dataCompleter = new Completer();
   Stream<List<int>> _stream;
@@ skipping 679 matching lines @@
   Stream<List<int>> bind(Stream<List<int>> stream) {
     return new Stream<List<int>>.eventTransformed(
         stream,
         (EventSink outSink) => new _BufferTransformerSink(outSink));
   }
 }
 
 
 class _HttpResponse extends _HttpOutboundMessage<HttpResponse>
     implements HttpResponse {
-  int statusCode = 200;
+  int _statusCode = 200;
   String _reasonPhrase;
   List<Cookie> _cookies;
   _HttpRequest _httpRequest;
   Duration _deadline;
   Timer _deadlineTimer;
 
   _HttpResponse(Uri uri,
                 String protocolVersion,
                 _HttpOutgoing outgoing,
                 String serverHeader)
       : super(uri, protocolVersion, outgoing) {
     if (serverHeader != null) headers.set('Server', serverHeader);
   }
 
   List<Cookie> get cookies {
     if (_cookies == null) _cookies = new List<Cookie>();
     return _cookies;
   }
 
+  int get statusCode => _statusCode;
+  void set statusCode(int statusCode) {
+    if (_headersWritten) throw new StateError("Header already sent");
+    _statusCode = statusCode;
+  }
+
   String get reasonPhrase => _findReasonPhrase(statusCode);
   void set reasonPhrase(String reasonPhrase) {
     if (_headersWritten) throw new StateError("Header already sent");
     _reasonPhrase = reasonPhrase;
   }
 
   Future redirect(Uri location, {int status: HttpStatus.MOVED_TEMPORARILY}) {
     if (_headersWritten) throw new StateError("Header already sent");
     statusCode = status;
     headers.set("Location", location.toString());
@@ skipping 1775 matching lines @@
   final Uri 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)';
 }