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

pkg/serialization/lib/src/mirrors_helpers.dart

Index: pkg/serialization/lib/src/mirrors_helpers.dart
diff --git a/pkg/serialization/lib/src/mirrors_helpers.dart b/pkg/serialization/lib/src/mirrors_helpers.dart
index cc57ecdb35d5a1b9eb9f75b73c2868061af28ed4..078aea0e8d1f653b8e0ded882c91ce1603cb30f7 100644
--- a/pkg/serialization/lib/src/mirrors_helpers.dart
+++ b/pkg/serialization/lib/src/mirrors_helpers.dart
@@ -57,17 +57,4 @@ List<MethodMirror> publicGettersWithMatchingSetters(ClassMirror mirror) {
  * 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;