Make argument to StringSink.writeln optional.

sdk/lib/io/http_impl.dart

Index: sdk/lib/io/http_impl.dart
diff --git a/sdk/lib/io/http_impl.dart b/sdk/lib/io/http_impl.dart
index 337c02fcd57e7d2ebe43345b19952d5d104ce987..1f794570966bd9fc7259047a629568abcc8b79e3 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -379,7 +379,7 @@ abstract class _HttpOutboundMessage<T> implements IOSink {
     for (Object obj in objects) write(obj);
   }
 
-  void writeln(Object obj) {
+  void writeln([Object obj = ""]) {
     write(obj);
     write("\n");
   }
@@ -1581,7 +1581,7 @@ class _DetachedSocket extends Stream<List<int>> implements Socket {
 
   void write(Object obj) => _socket.write(obj);
 
-  void writeln(Object obj) => _socket.writeln(obj);
+  void writeln([Object obj = ""]) => _socket.writeln(obj);
 
   void writeCharCode(int charCode) => _socket.writeCharCode(charCode);