Added back helper file accidentally deleted during test block 219 migration.

tests/lib/mirrors/disable_tree_shaking_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.
-
-// Ensure that reflection works on methods that would otherwise be
-// tree-shaken away.
-
-library lib;
-
-@MirrorsUsed(targets: "lib")
-import "dart:mirrors";
-
-class Foo {
-  Foo();
-  foo() => 42;
-}
-
-main() {
-  // Do NOT instantiate Foo.
-  var m = reflectClass(Foo);
-  var instanceMirror = m.newInstance(new Symbol(''), []);
-  var result = instanceMirror.invoke(new Symbol('foo'), []).reflectee;
-  if (result != 42) {
-    throw 'Expected 42, but got $result';
-  }
-}