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

pkg/intl/test/message_extraction/really_fail_extraction_test.dart

Index: pkg/intl/test/message_extraction/really_fail_extraction_test.dart
diff --git a/pkg/intl/test/message_extraction/really_fail_extraction_test.dart b/pkg/intl/test/message_extraction/really_fail_extraction_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..76f52519bc6ebf5c2e389e828704bbab7c80e2ed
--- /dev/null
+++ b/pkg/intl/test/message_extraction/really_fail_extraction_test.dart
@@ -0,0 +1,14 @@
+// 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);
+  });
+}