Normalize handling of the package root in the runner.

lib/src/runner/application_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 test.runner.application_exception;
+
+/// An expected exception caused by user-controllable circumstances.
+class ApplicationException implements Exception {
+  final String message;
+
+  ApplicationException(this.message);
+
+  String toString() => message;
+}