Avoid flaking in message_extraction tests by doing everything in a temp directory

pkg/intl/test/message_extraction/generated_messages_js_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.
-
-// This test is intended to be run when compiled to Javascript, though it will
-// also work if run from the VM, it just won't test anything different from
-// message_extraction_test. It runs the generated code without going through
-// the message extraction or translation, which can't be done in JS. This tells
-// us if the generated code works in JS.
-
-library generated_messages_js_test.dart;
-
-import 'package:unittest/unittest.dart';
-import 'sample_with_messages.dart' as sample;
-import 'print_to_list.dart';
-import 'verify_messages.dart';
-
-main() {
-  test("Test generated code running in JS", () =>
-    sample.main().then((_) => verifyResult(lines)));
-}