Add priority queue to package:collection.

pkg/collection/lib/collection.dart

Index: pkg/collection/lib/collection.dart
diff --git a/pkg/collection/lib/collection.dart b/pkg/collection/lib/collection.dart
index a6f192d8057e622eaf945f3e53fe12176ae51cae..d640071630f6d47c782ef4da53caa3a8ab45cd34 100644
--- a/pkg/collection/lib/collection.dart
+++ b/pkg/collection/lib/collection.dart
@@ -11,6 +11,7 @@
  *                      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.
  */
@@ -19,4 +20,5 @@ library dart.pkg.collection;
 export "algorithms.dart";
 export "equality.dart";
 export "iterable_zip.dart";
+export "priority_queue.dart";
 export "wrappers.dart";