Add mirrors tests

test/codegen/lib/mirrors/library_metadata2_lib1.dart

Index: test/codegen/lib/mirrors/library_metadata2_lib1.dart
diff --git a/test/codegen/language/hidden_import_lib.dart b/test/codegen/lib/mirrors/library_metadata2_lib1.dart
similarity index 77%
copy from test/codegen/language/hidden_import_lib.dart
copy to test/codegen/lib/mirrors/library_metadata2_lib1.dart
index c9bda07bc3138358f6bb9135ba2846ac3f8186ec..75149b362cf1e9803301c1695339c392496d7dcf 100644
--- a/test/codegen/language/hidden_import_lib.dart
+++ b/test/codegen/lib/mirrors/library_metadata2_lib1.dart
@@ -2,6 +2,9 @@
 // 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.
 
-library hidden_import_lib;
+@MyConst()
+library lib1;
 
-class Future {}
+class MyConst {
+  const MyConst();
+}