Update Set.difference to take a Set<Object>.

lib/src/typed_wrappers.dart

 // Copyright (c) 2016, 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.
 
 import "dart:collection";
 import "dart:math" as math;
 
 import "wrappers.dart";
 
 typedef F _UnaryFunction<E, F>(E argument);
@@ skipping 208 matching lines @@
   void addAll(Iterable<E> elements) {
     _setBase.addAll(elements);
   }
 
   void clear() {
     _setBase.clear();
   }
 
   bool containsAll(Iterable<Object> other) => _setBase.containsAll(other);
 
-  Set<E> difference(Set<E> other) =>
+  Set<E> difference(Set<Object> other) =>
       new TypeSafeSet<E>(_setBase.difference(other));
 
   Set<E> intersection(Set<Object> other) =>
       new TypeSafeSet<E>(_setBase.intersection(other));
 
   E lookup(Object element) => _setBase.lookup(element) as E;
 
   bool remove(Object value) => _setBase.remove(value);
 
   void removeAll(Iterable<Object> elements) {
@@ skipping 100 matching lines @@
   int get length => _base.length;
 
   V putIfAbsent(K key, V ifAbsent()) => _base.putIfAbsent(key, ifAbsent) as V;
 
   V remove(Object key) => _base.remove(key) as V;
 
   Iterable<V> get values => new TypeSafeIterable<V>(_base.values);
 
   String toString() => _base.toString();
 }