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

pkg/intl/test/message_extraction/run_and_verify.dart

Index: pkg/intl/test/message_extraction/run_and_verify.dart
diff --git a/tests/language/export_ambiguous_main_a.dart b/pkg/intl/test/message_extraction/run_and_verify.dart
similarity index 62%
copy from tests/language/export_ambiguous_main_a.dart
copy to pkg/intl/test/message_extraction/run_and_verify.dart
index 4a7b820ac658b7a8fa19066d6e4302e69289aa7c..c283f2c69a8462aa14f8e446d44bede1b231d627 100644
--- a/tests/language/export_ambiguous_main_a.dart
+++ b/pkg/intl/test/message_extraction/run_and_verify.dart
@@ -1,9 +1,11 @@
 // 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);
+}