Get rid of the polyfill identity set implementation in favour of a

pkg/serialization/lib/src/mirrors_helpers.dart

 // Copyright (c) 2012, 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.
 
 /**
  * Provides some additional convenience methods on top of the basic mirrors
  */
 library mirrors_helpers;
 
 // Import and re-export mirrors here to minimize both dependence on mirrors
@@ skipping 39 matching lines @@
   var setters = mirror.setters;
   return publicGetters(mirror).filter((each) =>
     setters["${each.simpleName}="] != null);
 }
 
 /**
  * A particularly bad case of polyfill, because we cannot yet use type names
  * as literals, so we have to be passed an instance and then extract a
  * ClassMirror from that. Given a horrible name as an extra reminder to fix it.
  */
-ClassMirror turnInstanceIntoSomethingWeCanUse(x) => reflect(x).type;
-
-/**
- * This is polyfill because we can't hash ClassMirror right now. We
- * don't bother implementing most of its methods because we don't need them.
- */
-// TODO(alanknight): Remove this when you can hash mirrors directly
-class ClassMirrorWrapper implements ClassMirror {
-  ClassMirror mirror;
-  ClassMirrorWrapper(this.mirror);
-  get simpleName => mirror.simpleName;
-  get hashCode => simpleName.hashCode;
-  operator ==(x) => x is ClassMirror && simpleName == x.simpleName;
-}
+ClassMirror turnInstanceIntoSomethingWeCanUse(x) => reflect(x).type;