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

tests/standalone/io/test_utils.dart

Index: tests/standalone/io/test_utils.dart
diff --git a/tests/standalone/io/test_utils.dart b/tests/standalone/io/test_utils.dart
index c8bf0cef0efb9538bebd3d566756e32997a341f3..ace66035c4f3129ac8c1c4ebe440d4d773d41908 100644
--- a/tests/standalone/io/test_utils.dart
+++ b/tests/standalone/io/test_utils.dart
@@ -13,15 +13,21 @@ Future<int> freeIPv4AndIPv6Port() async {
   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();
 }