Make argument to StringSink.writeln optional.

pkg/http/test/safe_http_server.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.
 
 library safe_http_server;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:uri';
 
@@ skipping 122 matching lines @@
   HttpConnectionInfo get connectionInfo => _inner.connectionInfo;
   void writeBytes(List<int> data) => _inner.writeBytes(data);
   Future<HttpResponse> consume(Stream<List<int>> stream) =>
     _inner.consume(stream);
   Future<HttpResponse> writeStream(Stream<List<int>> stream) =>
     _inner.writeStream(stream);
   void close() => _inner.close();
   void write(Object obj) => _inner.write(obj);
   void writeAll(Iterable objects) => _inner.writeAll(objects);
   void writeCharCode(int charCode) => _inner.writeCharCode(charCode);
-  void writeln(Object obj) => _inner.writeln(obj);
+  void writeln([Object obj = ""]) => _inner.writeln(obj);
 }