Make argument to StringSink.writeln optional.

sdk/lib/io/io_sink.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;
 
 /**
  * Helper class to wrap a [StreamConsumer<List<int>, T>] and provide
  * utility functions for writing to the StreamConsumer directly. The
  * [IOSink] buffers the input given by [write], [writeAll], [writeln],
@@ skipping 80 matching lines @@
       }
     }
     if (string.isEmpty) return;
     writeBytes(_encodeString(string, _encoding));
   }
 
   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) {
     if (_isBound) {
@@ skipping 113 matching lines @@
         },
         onError: _controller.addError);
     if (_paused) _pause();
     if (unbind) {
       return _writeStreamCompleter.future;
     } else {
       return _pipeFuture;
     }
   }
 }