Make argument to StringSink.writeln optional.

sdk/lib/_internal/compiler/implementation/code_buffer.dart

 // Copyright (c) 2012, 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 dart2js;
 
 class CodeBuffer implements StringBuffer {
   StringBuffer buffer;
   List<CodeBufferMarker> markers;
   int lastBufferOffset = 0;
@@ skipping 26 matching lines @@
     return this;
   }
 
   CodeBuffer writeAll(Iterable<Object> objects) {
     for (var object in objects) {
       write(object);
     }
     return this;
   }
 
-  CodeBuffer writeln(var object) {
+  CodeBuffer writeln([var object = ""]) {
     return write(object).write("\n");
   }
 
   CodeBuffer addBuffer(CodeBuffer other) {
     if (other.markers.length > 0) {
       CodeBufferMarker firstMarker = other.markers[0];
       int offsetDelta =
           buffer.length + firstMarker.offsetDelta - lastBufferOffset;
       markers.add(new CodeBufferMarker(offsetDelta,
                                        firstMarker.sourcePosition));
@@ skipping 52 matching lines @@
     });
   }
 }
 
 class CodeBufferMarker {
   final int offsetDelta;
   final sourcePosition;
 
   CodeBufferMarker(this.offsetDelta, this.sourcePosition);
 }