Add mirrors tests

test/codegen/lib/mirrors/library_metadata_test.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+@string @symbol
+library test.library_metadata_test;
+
+import 'dart:mirrors';
+
+import 'metadata_test.dart';
+
+main() {
+  MirrorSystem mirrors = currentMirrorSystem();
+  checkMetadata(mirrors.findLibrary(#test.library_metadata_test),
+                [string, symbol]);
+  checkMetadata(mirrors.findLibrary(#test.metadata_test), []);
+}