Make argument to StringSink.writeln optional.

sdk/lib/core/string_buffer.dart

 // Copyright (c) 2011, 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;
 
 /**
  * The StringBuffer class is useful for concatenating strings
  * efficiently. Only on a call to [toString] are the strings
  * concatenated to a single String.
@@ skipping 15 matching lines @@
   /// Adds the contents of [obj], converted to a string, to the buffer.
   external void write(Object obj);
 
   /// Adds the string representation of [charCode] to the buffer.
   external void writeCharCode(int charCode);
 
   void writeAll(Iterable objects) {
     for (Object obj in objects) write(obj);
   }
 
-  void writeln(Object obj) {
+  void writeln([Object obj = ""]) {
     write(obj);
     write("\n");
   }
 
   /**
    * Clears the string buffer.
    */
   external void clear();
 
   /// Returns the contents of buffer as a concatenated string.
   external String toString();
 }