Add priority queue to package:collection.

sdk/lib/core/comparable.dart

 // Copyright (c) 2011, 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.
 
 part of dart.core;
 
 /**
  * The signature of a generic comparison function.
  *
  * A comparison function represents an ordering on a type of objects.
@@ skipping 45 matching lines @@
    * Returns a value like a [Comparator] when comparing [:this:] to [other].
    *
    * May throw an [ArgumentError] if [other] is of a type that
    * is not comparable to [:this:].
    */
   int compareTo(T other);
 
   /**
    * A [Comparator] that compares one comparable to another.
    *
+   * It returns the result of `a.compareTo(b)`.
+   *
    * This utility function is used as the default comparator
    * for ordering collections, for example in the [List] sort function.
    */
   static int compare(Comparable a, Comparable b) => a.compareTo(b);
 }