Add test for native extensions that copies the extension to a test directory and runs it.

tests/standalone/io/test_extension_tester.dart

 // Copyright (c) 2012, 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("test_extension_test");
 
 #import('test_extension.dart');
 
 main() {
   Expect.equals('cat 13', new Cat(13).toString(), 'new Cat(13).toString()');
 
   Expect.equals(3, Cat.ifNull(null, 3), 'Cat.ifNull(null, 3)');
   Expect.equals(4, Cat.ifNull(4, null), 'Cat.ifNull(4, null)');
   Expect.equals(5, Cat.ifNull(5, 9), 'Cat.ifNull(5, 9)');
   Expect.isNull(Cat.ifNull(null, null), 'Cat.ifNull(null, null)');
 }