Temporarily re-add a library tag.

test/io.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.
 
+// 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';
 
 import 'package:path/path.dart' as p;
 import 'package:scheduled_test/descriptor.dart' as d;
 import 'package:scheduled_test/scheduled_process.dart';
 import 'package:scheduled_test/scheduled_stream.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:test/src/util/io.dart';
 
@@ skipping 142 matching lines @@
     var match;
     while (match == null) {
       var line = await pub.stdout.next();
       match = _servingRegExp.firstMatch(line);
     }
     _pubServePortCompleter.complete(int.parse(match[1]));
   }, "waiting for pub serve to emit its port number");
 
   return pub;
 }