Add the index information to network interface

tests/standalone/io/network_interface_test.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.
 
 import 'dart:io';
 
 import "package:expect/expect.dart";
 
 
 void testListLoopback() {
@@ skipping 15 matching lines @@
   NetworkInterface.list(includeLinkLocal: false).then((list) {
     for (var i in list) {
       for (var a in i.addresses) {
         Expect.isFalse(a.isLinkLocal);
       }
     }
   });
 }
 
 
+void testListIndex() {
+  var set = new Set();
+  NetworkInterface.list(includeLoopback: true).then((list) {
+    for (var i in list) {
+      Expect.isNotNull(i.index);
+      Expect.isFalse(set.contains(i.index));
+      set.add(i.index);
+      Expect.isTrue(set.contains(i.index));
+    }
+  });
+}
+
+
 void main() {
   testListLoopback();
   testListLinkLocal();
+  testListIndex();
 }