Make argument to StringSink.writeln optional.

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 361 matching lines @@
       }
     }
     if (string.isEmpty) return;
     _ioSink.write(string);
   }
 
   void writeAll(Iterable objects) {
     for (Object obj in objects) write(obj);
   }
 
-  void writeln(Object obj) {
+  void writeln([Object obj = ""]) {
     write(obj);
     write("\n");
   }
 
   void writeCharCode(int charCode) {
     write(new String.fromCharCode(charCode));
   }
 
   void writeBytes(List<int> data) {
     _writeHeaders();
@@ skipping 1181 matching lines @@
   }
 
   Encoding get encoding => _socket.encoding;
 
   void set encoding(Encoding value) {
     _socket.encoding = value;
   }
 
   void write(Object obj) => _socket.write(obj);
 
-  void writeln(Object obj) => _socket.writeln(obj);
+  void writeln([Object obj = ""]) => _socket.writeln(obj);
 
   void writeCharCode(int charCode) => _socket.writeCharCode(charCode);
 
   void writeAll(Iterable objects) => _socket.writeAll(objects);
 
   void writeBytes(List<int> bytes) => _socket.writeBytes(bytes);
 
   Future<Socket> consume(Stream<List<int>> stream) {
     return _socket.consume(stream);
   }
@@ skipping 125 matching lines @@
 
 
 class _RedirectInfo implements RedirectInfo {
   const _RedirectInfo(int this.statusCode,
                       String this.method,
                       Uri this.location);
   final int statusCode;
   final String method;
   final Uri location;
 }