Add mirrors tests

test/codegen/lib/mirrors/unused_mirrors4_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.
+
+// Tests that MirrorsUsed without usage of the static field does not crash
+// dart2js.
+
+@MirrorsUsed(targets: const ["foo"])
+import 'dart:mirrors';
+
+Bar get foo => new Bar();
+
+typedef int FunctionTypeDef();
+
+class Bar {
+  final FunctionTypeDef gee;
+  Bar() : gee = (() => 499);
+}
+
+main() {
+  // Don't do anything.
+}