Change Set.difference API to accept Set<Object>.

pkg/compiler/lib/src/util/emptyset.dart

 // Copyright (c) 2014, 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.
 
 library dart2js.util.emptyset;
 
 import 'dart:collection' show IterableBase;
 
 class ImmutableEmptySet<E> extends IterableBase<E> implements Set<E> {
   const ImmutableEmptySet();
@@ skipping 14 matching lines @@
   void retainAll(Iterable<E> elements) {}
   void retainWhere(bool test(E element)) {}
   void forEach(void action(E element)) {}
   void clear() {}
 
   bool contains(E element) => false;
   bool containsAll(Iterable<E> other) => other.isEmpty;
 
   Set<E> union(Set<E> other) => new Set.from(other);
   Set<E> intersection(Set<E> other) => this;
-  Set<E> difference(Set<E> other) => this;
+  Set<E> difference(Set<Object> other) => this;
   Set<E> toSet() => new Set();
 }
 
 class _EmptySetIterator<E> implements Iterator<E> {
   const _EmptySetIterator();
 
   E get current => null;
   bool moveNext() => false;
 }