Stop using the Hashable interface.

lib/core/set.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.
 
 /**
  * This class is the public interface of a set. A set is a collection
  * without duplicates.
  */
 abstract class Set<E> extends Collection<E> {
   factory Set() => new HashSetImplementation<E>();
@@ skipping 51 matching lines @@
    */
   Set<E> intersection(Collection<E> other);
 
   /**
    * Removes all elements in the set.
    */
   void clear();
 
 }
 
-abstract class HashSet<E extends Hashable> extends Set<E> {
+abstract class HashSet<E> extends Set<E> {
   factory HashSet() => new HashSetImplementation<E>();
 
   /**
    * Creates a [Set] that contains all elements of [other].
    */
   factory HashSet.from(Iterable<E> other) =>
       new HashSetImplementation<E>.from(other);
 }