Add mirrors tests

test/codegen/lib/mirrors/library_imports_metadata.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 library_imports_metadata;
+
+@m1
+import 'dart:mirrors' as mirrors;
+
+@m2 @m3
+import 'dart:collection';
+
 import 'dart:async';
-import 'async_helper_lib.dart' as async;
 
-class A {
-  async.async get async => null;
-}
-
-async.async topLevel() => null;
-
-main() {
-  var a = new A();
-  var b = a.async;
-  var c = topLevel();
-}
+const m1 = const Object();
+const m2 = const Object();
+const m3 = const Object();