Reload native extensions when starting from a snapshot.

samples/sample_extension/test/sample_extension_script_snapshot_test.dart

 // Copyright (c) 2016, 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.
+//
+// Dart test program for testing native extensions.
 
-import "package:expect/expect.dart";
-
-class Bar<T> {
-}
-
-class Foo<T> extends Bar<Foo<T>> {
-}
+import 'sample_extension_test_helper.dart';
 
 void main() {
-  print(new Foo<int>());
+  testNativeExtensions("script");
 }