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

pkg/intl/test/message_extraction/run_and_verify.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.
+library verify_and_run;
 
-library export_ambiguous_main_a;
+import 'sample_with_messages.dart' as sample;
+import 'verify_messages.dart';
 
 main() {
-  print('export_ambiguous_main_a');
-}
+  sample.main().then(verifyResult);
+}