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

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

 // Copyright (c) 2013, 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.setlet;
 
 import 'dart:collection' show IterableBase;
 
 class Setlet<E> extends IterableBase<E> implements Set<E> {
   static const _MARKER = const _SetletMarker();
@@ skipping 232 matching lines @@
   clear() {
     _contents = _MARKER;
     _extra = null;
   }
 
   Set<E> union(Set<E> other) => new Set<E>.from(this)..addAll(other);
 
   Setlet<E> intersection(Set<E> other) =>
       new Setlet<E>.from(this.where((e) => other.contains(e)));
 
-  Setlet<E> difference(Set<E> other) =>
+  Setlet<E> difference(Set<Object> other) =>
       new Setlet<E>.from(this.where((e) => !other.contains(e)));
 
   Setlet<E> toSet() {
     Setlet<E> result = new Setlet<E>();
     if (_extra == null) {
       result._contents = _contents;
     } else if (_extra == _MARKER) {
       result._extra = _MARKER;
       result._contents = _contents.toSet();
     } else {
@@ skipping 42 matching lines @@
       if (Setlet._MARKER != candidate) {
         _current = candidate;
         _remaining--;
         return true;
       }
     }
     _current = null;
     return false;
   }
 }