Fix incorrect signature in socket.dart.

R=vegorov@google.com

Review-Url: https://codereview.chromium.org/2752153003 .
diff --git a/sdk/lib/io/socket.dart b/sdk/lib/io/socket.dart
index 0a112d5..1f733ba 100644
--- a/sdk/lib/io/socket.dart
+++ b/sdk/lib/io/socket.dart
@@ -695,7 +695,7 @@
    * 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 @@
    * 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]);
 }