Make argument to StringSink.writeln optional.

sdk/lib/core/string_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.core;
 
 abstract class StringSink {
 
   /**
    * Converts [obj] to a String by invoking `toString` and adds the result to
    * `this`.
    */
   void write(Object obj);
 
   /**
    * Iterates over the given [objects] and [write]s them in sequence.
    */
   void writeAll(Iterable objects);
 
   /**
    * Converts [obj] to a String by invoking `toString` and adds the result to
    * `this`. Then adds a new line.
    */
-  void writeln(Object obj);
+  void writeln([Object obj = ""]);
 
   /**
    * Writes the [charCode] to `this`.
    *
    * This method is equivalent to `write(new String.fromCharCode(charCode))`.
    */
   void writeCharCode(int charCode);
 }