Add isWithin to pkg/path.

pkg/path/test/utils.dart

Index: pkg/path/test/utils.dart
diff --git a/pkg/path/test/utils.dart b/pkg/path/test/utils.dart
new file mode 100644
index 0000000000000000000000000000000000000000..1730798f09479c06275a190bcce69f0e355daf04
--- /dev/null
+++ b/pkg/path/test/utils.dart
@@ -0,0 +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 path.test.utils;
+
+import "package:unittest/unittest.dart";
+import "package:path/path.dart" as path;
+
+/// A matcher for a closure that throws a [path.PathException].
+final throwsPathException = throwsA(new isInstanceOf<path.PathException>());