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

tests/lib/mirrors/empty_test.dart

Index: tests/lib/mirrors/empty_test.dart
diff --git a/tests/compiler/dart2js/dart2js_batch2_run.dart b/tests/lib/mirrors/empty_test.dart
similarity index 60%
copy from tests/compiler/dart2js/dart2js_batch2_run.dart
copy to tests/lib/mirrors/empty_test.dart
index aee55ef18e5fa77c47bcbf53f5c16125299ed77a..506cdfbef3af725e005a3841000c65a9441b2f76 100644
--- a/tests/compiler/dart2js/dart2js_batch2_run.dart
+++ b/tests/lib/mirrors/empty_test.dart
@@ -2,9 +2,10 @@
 // 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.
 
-@notExisting
-var x;
+import 'dart:mirrors';
+import 'empty.dart';
 
 main() {
-  print(x);
+  var empty = currentMirrorSystem().findLibrary(#empty);
+  print(empty.location);  // Used to crash VM.
 }