Add priority queue to package:collection.

pkg/collection/README.md

 Helper libraries for working with collections.
 
 The `collection` package contains a number of separate libraries
 with utility functions and classes that makes working with collections easier.
 
 ## Using
 
 The `collection` package can be imported as separate libraries, or
 in totality:
 
+    import 'package:collection/algorithms.dart';
     import 'package:collection/equality.dart';
-    import 'package:collection/algorithms.dart';
+    import 'package:collection/iterable_zip.dart';
+    import 'package:collection/priority_queue.dart';
     import 'package:collection/wrappers.dart';
 
 or
 
     import 'package:collection/collection.dart';
 
+## Algorithms
+
+The algorithms library contains functions that operate on lists.
+
+It contains ways to shuffle a `List`, do binary search on a sorted `List`, and
+various sorting algorithms.
+
+
 ## Equality
 
 The equality library gives a way to specify equality of elements and
 collections.
 
 Collections in Dart have no inherent equality. Two sets are not equal, even
 if they contain exactly the same objects as elements.
 
 The equality library provides a way to say define such an equality. In this
 case, for example, `const SetEquality(const IdentityEquality())` is an equality
 that considers two sets equal exactly if they contain identical elements.
 
-The library provides ways to define equalities on `Iterable`s, `List`s, `Set`s, and
-`Map`s, as well as combinations of these, such as:
+The library provides ways to define equalities on `Iterable`s, `List`s, `Set`s,
+and `Map`s, as well as combinations of these, such as:
 
     const MapEquality(const IdentityEquality(), const ListEquality());
 
-This equality considers maps equal if they have identical keys, and the corresponding values are lists with equal (`operator==`) values.
+This equality considers maps equal if they have identical keys, and the
+corresponding values are lists with equal (`operator==`) values.
 
-## Algorithms
 
-The algorithms library contains functions that operate on lists.
+## Iterable Zip
 
-It contains ways to shuffle a `List`, do binary search on a sorted `List`, and
-some different sorting algorithms.
+Utilities for "zipping" a list of iterables into an iterable of lists.
+
+
+## Priority Queue
+
+An interface and implemention of a priority queue.
 
 
 ## Wrappers
 
 The wrappers library contains classes that "wrap" a collection.
 
 A wrapper class contains an object of the same type, and it forwards all
 methods to the wrapped object.
 
 Wrapper classes can be used in various ways, for example to restrict the type
 of an object to that of a supertype, or to change the behavior of selected
 functions on an existing object.