Add mirrors tests

test/codegen/lib/mirrors/unused_mirrors_used_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.
+
+// Regression test.
+// The MirrorsUsed annotation made dart2js to mark List as needed for reflection
+// but tree-shaking wasn't turned off (since there is no actual mirror use)
+// which led to a broken output.
+
+@MirrorsUsed(targets: "List")
+import 'dart:mirrors';
+
+void main() {
+  var l = new List<int>();
+  var f = l.retainWhere;
+  f((x) => true);
+}