Fixes NetworkInterface.list crash on Android

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 29 matching lines @@
       Expect.isNotNull(i.index);
       Expect.isFalse(set.contains(i.index));
       set.add(i.index);
       Expect.isTrue(set.contains(i.index));
     }
   });
 }
 
 
 void main() {
+  if (!NetworkInterface.listSupported) {
+    return;
+  }
   testListLoopback();
   testListLinkLocal();
   testListIndex();
 }