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

pkg/compiler/lib/src/helpers/expensive_set.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.
 
 import 'dart:collection';
 
 /**
  * The expensive set is a data structure useful for tracking down
  * excessive memory usage due to large sets. It acts as an ordinary
  * hash set, but it uses 10 times more memory (by default).
@@ skipping 83 matching lines @@
         if (other.contains(element)) result.add(element);
       }
     }
     return result;
   }
 
   Set<E> union(Set<E> other) {
     return _newSet()..addAll(this)..addAll(other);
   }
 
-  Set<E> difference(Set<E> other) {
+  Set<E> difference(Set<Object> other) {
     Set<E> result = _newSet();
     for (E element in this) {
       if (!other.contains(element)) result.add(element);
     }
     return result;
   }
 
   void retainAll(Iterable objectsToRetain) {
     Set retainSet;
     if (objectsToRetain is Set) {
       retainSet = objectsToRetain;
     } else {
       retainSet = objectsToRetain.toSet();
     }
     retainWhere(retainSet.contains);
   }
 
   Set<E> toSet() {
     var result = new ExpensiveSet<E>(_sets.length);
     for (int i = 0; i < _sets.length; i++) {
       result._sets[i] = _sets[i].toSet();
     }
     return result;
   }
 
   String toString() => "expensive(${_sets[0]}x${_sets.length})";
 }