Add mirrors tests

test/codegen/lib/mirrors/metadata_const_map_test.dart

+// Copyright (c) 2014, 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.
+
+// Regression test for issue 20776. Tests that the needed classes for the
+// constant map in the metadata are generated.
+
+library lib;
+
+@MirrorsUsed(targets: 'lib')
+import 'dart:mirrors';
+
+class C {
+  final x;
+  const C(this.x);
+}
+
+@C(const {'foo': 'bar' })
+class A {}
+
+
+main() {
+  print(reflectClass(A).metadata);
+}