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

tests/lib_2/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.
 
 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';
   }
 }