Fix flaky timeouts of socket_ipv6_test.

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 2dbaf977e4033d0d8fb66c40086e49f6d357bc2f..f1bd08062c3db6906536b53c8ceefe4d37c53b23 100644
--- a/tests/standalone/io/test_utils.dart
+++ b/tests/standalone/io/test_utils.dart
@@ -26,3 +26,17 @@ Future retry(Future fun(), {int maxCount: 10}) async {
   return await fun();
 }
 
+
+retrySync(fun, {int maxCount: 10}) {
+  for (int i = 0; i < maxCount; i++) {
+    try {
+      // If there is no exception this will simply return, otherwise we keep
+      // trying.
+      return fun();
+    } catch (_) {}
+    print("Failed to execute test closure in attempt $i "
+          "(${maxCount - i} retries left).");
+  }
+  return fun();
+}
+