Improve error message in flaky test

tests/corelib/list_unmodifiable_test.dart

Index: tests/corelib/list_unmodifiable_test.dart
diff --git a/tests/corelib/list_unmodifiable_test.dart b/tests/corelib/list_unmodifiable_test.dart
index e2f82e7bd08c194328d76539ffc4b2da0050069b..2989ea98cb55a76c979d86cf2b6273a62c292d3f 100644
--- a/tests/corelib/list_unmodifiable_test.dart
+++ b/tests/corelib/list_unmodifiable_test.dart
@@ -107,7 +107,9 @@ class Test<E> {
         try {
           op(list);
         } catch (e2) {
-          Expect.equals(e.runtimeType, e2.runtimeType);
+          Expect.equals(e.runtimeType, e2.runtimeType,
+              "$name :: $opName threw different exceptions: "
+              "${e.runtimeType} vs ${e2.runtimeType}");
           return;
         }
         Expect.fail("$name-$opName didn't throw, expected: $e");