Add mirrors tests

test/codegen/lib/mirrors/deferred_mirrors_metatarget_lib.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.
+
+library lib;
+
+@MirrorsUsed(metaTargets: "MetaTarget")
+import "dart:mirrors";
+
+class MetaTarget {
+  const MetaTarget();
+}
+
+@MetaTarget()
+class A {
+  String toString() => "A";
+}
+
+String foo() {
+  ClassMirror a = currentMirrorSystem().findLibrary(#lib).declarations[#A];
+  return a.newInstance(const Symbol(""), []).invoke(#toString, []).reflectee;
+}