Stop using the Hashable interface.

tests/corelib/set_iterator_test.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.
 
-class FixedHashCode implements Hashable {
+class FixedHashCode {
   final int _hashCode;
   const FixedHashCode(this._hashCode);
   int hashCode() { return _hashCode; }
 }
 
 class SetIteratorTest {
   static testMain() {
     testSmallSet();
     testLargeSet();
     testEmptySet();
@@ skipping 124 matching lines @@
         Expect.equals(true, x === element);
       }
       Expect.equals(true, foundIt);
     }
   }
 }
 
 main() {
   SetIteratorTest.testMain();
 }