Add priority queue to package:collection.

pkg/collection/lib/collection.dart

 // 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.
 
 /**
  * Exports all the individual parts of the collection-helper library.
  *
  * The sub-libraries of this package are:
  *
  * - `algorithms.dart`: Algorithms that work on lists (shuffle, binary search
  *                      and various sorting algorithms).
  * - `equality.dart`: Different notions of equality of collections.
  * - `iterable_zip.dart`: Combining multiple iterables into one.
+ * - `priority_queue.dart`: Priority queue type and implementations.
  * - `wrappers.dart`: Wrapper classes that delegate to a collection object.
  *                    Includes unmodifiable views of collections.
  */
 library dart.pkg.collection;
 
 export "algorithms.dart";
 export "equality.dart";
 export "iterable_zip.dart";
+export "priority_queue.dart";
 export "wrappers.dart";