Make argument to StringSink.writeln optional.

runtime/bin/socket_patch.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.
 
 patch class RawServerSocket  {
   /* patch */ static Future<RawServerSocket> bind([String address = "127.0.0.1",
                                                    int port = 0,
                                                    int backlog = 0]) {
     return _RawServerSocket.bind(address, port, backlog);
   }
@@ skipping 763 matching lines @@
   }
 
   Encoding get encoding => _sink.encoding;
 
   void set encoding(Encoding value) {
     _sink.encoding = value;
   }
 
   void write(Object obj) => _sink.write(obj);
 
-  void writeln(Object obj) => _sink.writeln(obj);
+  void writeln([Object obj = ""]) => _sink.writeln(obj);
 
   void writeCharCode(int charCode) => _sink.writeCharCode(charCode);
 
   void writeAll(Iterable objects) => _sink.writeAll(objects);
 
   void writeBytes(List<int> bytes) => _sink.writeBytes(bytes);
 
   Future<Socket> consume(Stream<List<int>> stream) {
     return _sink.consume(stream);
   }
@@ skipping 150 matching lines @@
     _raw.onBadCertificate = callback;
   }
 
   X509Certificate get peerCertificate {
     if (_raw == null) {
      throw new StateError("peerCertificate called on destroyed SecureSocket");
     }
     return _raw.peerCertificate;
   }
 }