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

pkg/intl/test/message_extraction/really_fail_extraction_test.dart

+// Copyright (c) 2014, 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 really_fail_extraction_test;
+
+import "failed_extraction_test.dart";
+import "package:unittest/unittest.dart";
+
+main() {
+  test("Expect failure because warnings are errors", () {
+    runTestWithWarnings(warningsAreErrors: true, expectedExitCode: 1);
+  });
+}