Gardening: improve debug output in standalone/io/socket_ipv6_test.dart.

tests/standalone/io/test_utils.dart

 // Copyright (c) 2016, 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:async';
 import 'dart:io';
 
 Future<int> freeIPv4AndIPv6Port() async {
   var socket =
     await ServerSocket.bind(InternetAddress.ANY_IP_V6, 0, v6Only: false);
   int port = socket.port;
   await socket.close();
   return port;
 }
 
+int lastRetryId = 0;
+
 Future retry(Future fun(), {int maxCount: 10}) async {
+  final int id = lastRetryId++;
   for (int i = 0; i < maxCount; i++) {
     try {
       // If there is no exception this will simply return, otherwise we keep
       // trying.
       return await fun();
-    } catch (_) {}
-    print("Failed to execute test closure in attempt $i "
-          "(${maxCount - i} retries left).");
+    } catch (e, stack) {
+      print("Failed to execute test closure (retry id: ${id}) in attempt $i "
+            "(${maxCount - i} retries left).");
+      print("Exception: ${e}");
+      print("Stacktrace: ${stack}");
+    }
   }
   return await fun();
 }