Add mirrors tests

test/codegen/lib/mirrors/library_exports_shown.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.
 
-int get async { return 1; }
+library library_exports_shown;
 
-class A {
-  async() => null;
-}
+export 'library_imports_a.dart' show somethingFromA, somethingFromBoth;
+export 'library_imports_b.dart' show somethingFromB;
 
-main() {
-  var a = async;
-  var b = new A();
-  var c = b.async();
-}
+var somethingFromShown;