Temporarily re-add a library tag.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1578303003 .

test/io.dart

@@ -2,6 +2,10 @@
 // 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.
 
+// TODO(nweiz): Remove this tag when we can get [packageDir] working without it
+// (dart-lang/sdk#24022).
+library test.test.io;
+
 import 'dart:async';
 import 'dart:io';