Fix incorrect signature in socket.dart.

sdk/lib/io/socket.dart

 // Copyright (c) 2013, 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.io;
 
 
 /**
  * [InternetAddressType] is the type an [InternetAddress]. Currently,
  * IP version 4 (IPv4) and IP version 6 (IPv6) are supported.
@@ skipping 677 matching lines @@
    * The maximum length of the datagram that can be received is 65503 bytes.
    */
   Datagram receive();
 
   /**
    * Join a multicast group.
    *
    * 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.
    *
    * 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]);
 }
 
 
 class SocketException implements IOException {
   final String message;
   final OSError osError;
   final InternetAddress address;
   final int port;
 
   const SocketException(this.message, {this.osError, this.address, this.port});
@@ skipping 16 matching lines @@
     }
     if (address != null) {
       sb.write(", address = ${address.host}");
     }
     if (port != null) {
       sb.write(", port = $port");
     }
     return sb.toString();
   }
 }