Add writeHeaders argument to HttpResponse::detachSocket.

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 _OUTGOING_BUFFER_SIZE = 8 * 1024;
 
 class _HttpIncoming extends Stream<List<int>> {
   final int _transferLength;
@@ skipping 508 matching lines @@
     _reasonPhrase = reasonPhrase;
   }
 
   Future redirect(Uri location, {int status: HttpStatus.MOVED_TEMPORARILY}) {
     if (_outgoing.headersWritten) throw new StateError("Header already sent");
     statusCode = status;
     headers.set("location", location.toString());
     return close();
   }
 
-  Future<Socket> detachSocket() {
+  Future<Socket> detachSocket({bool writeHeaders: true}) {
     if (_outgoing.headersWritten) throw new StateError("Headers already sent");
     deadline = null;  // Be sure to stop any deadline.
     var future = _httpRequest._httpConnection.detachSocket();
-    var headersFuture = _outgoing.writeHeaders(drainRequest: false,
-                                               setOutgoing: false);
-    assert(headersFuture == null);
+    if (writeHeaders) {
+      var headersFuture = _outgoing.writeHeaders(drainRequest: false,
+                                                 setOutgoing: false);
+      assert(headersFuture == null);
+    } else {
+      // Imitate having written the headers.
+      _outgoing.headersWritten = true;
+    }
     // Close connection so the socket is 'free'.
     close();
     done.catchError((_) {
       // Catch any error on done, as they automatically will be
       // propagated to the websocket.
     });
     return future;
   }
 
   HttpConnectionInfo get connectionInfo => _httpRequest.connectionInfo;
@@ skipping 2070 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)';
 }