Add the index information to network interface

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 f1ceec534b7453aa5aa5b767f98e236e951f0f7a..2f789585e6ca4313485ed1cdc31a981035734921 100644
--- a/sdk/lib/io/socket.dart
+++ b/sdk/lib/io/socket.dart
@@ -145,6 +145,11 @@ abstract class NetworkInterface {
   String get name;
 
   /**
+   * Get the index of the [NetworkInterface].

[Anders Johnsen, 2013/11/29 08:35:10]

Is this available on IPv4 only interfaces?

[Søren Gjesse, 2013/11/29 10:01:20]

I think so. On windows it is part of the standard interface information
structure, and on Mac OS and Linux it just maps a interface name to an index.

I will try it out on non IPv6 adapters later.

+   */
+  String get index;
+
+  /**
    * Get a list of [InternetAddress]s currently bound to this
    * [NetworkInterface].
    */