Ensure libraries always have at least one top-level class and therefore at least one script.

tests/lib/mirrors/empty.dart

Index: tests/lib/mirrors/empty.dart
diff --git a/tests/compiler/dart2js_extra/deferred_fail_and_retry_lib.dart b/tests/lib/mirrors/empty.dart
similarity index 81%
copy from tests/compiler/dart2js_extra/deferred_fail_and_retry_lib.dart
copy to tests/lib/mirrors/empty.dart
index bf021a41818cb8a50e539b0689f4b8624792fd74..ee5f193a03cbc33674b250f5a6e5eca3c1566d5b 100644
--- a/tests/compiler/dart2js_extra/deferred_fail_and_retry_lib.dart
+++ b/tests/lib/mirrors/empty.dart
@@ -2,6 +2,5 @@
 // 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.
 
-foo() {
-  return "loaded";
-}
+// This library has no functions.
+library empty;