Run test tearDowns and clean up temporary directories when a signal is caught.

lib/src/backend/closed_exception.dart

+// Copyright (c) 2015, 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 unittest.backend.closed_exception;
+
+/// An exception thrown by various front-end methods when the test framework has
+/// been closed and a test must shut down as soon as possible.
+class ClosedException implements Exception {
+  ClosedException();
+
+  String toString() => "This test has been closed.";
+}