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

pkg/compiler/lib/src/helpers/debug_collection.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.
 
 typedef void DebugCallback(String methodName, var arg1, var arg2);
 
 class DebugMap<K, V> implements Map<K, V> {
   final Map<K, V> map;
   DebugCallback indexSetCallBack;
   DebugCallback putIfAbsentCallBack;
@@ skipping 243 matching lines @@
   void removeWhere(bool test(E element)) => set.removeWhere(test);
 
   void retainWhere(bool test(E element)) => set.retainWhere(test);
 
   bool containsAll(Iterable<Object> other) => set.containsAll(other);
 
   Set<E> intersection(Set<Object> other) => set.intersection(other);
 
   Set<E> union(Set<E> other) => set.union(other);
 
-  Set<E> difference(Set<E> other) => set.difference(other);
+  Set<E> difference(Set<Object> other) => set.difference(other);
 
   void clear() => set.clear();
 
   Set<E> toSet() => set.toSet();
 }
 
 /// Throws an exception if the runtime type of [object] is not in
 /// [runtimeTypes].
 ///
 /// Use this to gradually build the set of actual runtime values of [object]
@@ skipping 53 matching lines @@
     check(element, '$methodName: ');
   }
 
   void check(var element, [String text = '']) {
     String elementType = '${element.runtimeType}';
     if (runtimeTypes.contains(elementType)) return;
     throw '$name: $text$elementType'
         '${showObjects ? ' ($element)' : ''}';
   }
 }