Fix incorrect signature in socket.dart.

sdk/lib/io/socket.dart

Index: sdk/lib/io/socket.dart
diff --git a/sdk/lib/io/socket.dart b/sdk/lib/io/socket.dart
index 0a112d556e9ad3b7d29c6794c4e322bcb1f0337f..1f733ba617383c527a422cc324222bb2d3eae860 100644
--- a/sdk/lib/io/socket.dart
+++ b/sdk/lib/io/socket.dart
@@ -695,7 +695,7 @@ abstract class RawDatagramSocket extends Stream<RawSocketEvent> {
    * If an error occur when trying to join the multicast group an
    * exception is thrown.
    */
-  void joinMulticast(InternetAddress group, {NetworkInterface interface});
+  void joinMulticast(InternetAddress group, [NetworkInterface interface]);
 
   /**
    * Leave a multicast group.
@@ -703,7 +703,7 @@ abstract class RawDatagramSocket extends Stream<RawSocketEvent> {
    * If an error occur when trying to join the multicase group an
    * exception is thrown.
    */
-  void leaveMulticast(InternetAddress group, {NetworkInterface interface});
+  void leaveMulticast(InternetAddress group, [NetworkInterface interface]);
 }