SDK fixes

test/generated_sdk/lib/collection/set.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.
 
 /**
  * Base implementations of [Set].
  */
 part of dart.collection;
 
 /**
@@ skipping 15 matching lines @@
  */
 abstract class SetMixin<E> implements Set<E> {
   // This class reimplements all of [IterableMixin].
   // If/when Dart mixins get more powerful, we should just create a single
   // Mixin class from IterableMixin and the new methods of thisclass.
 
   bool add(E element);
 
   bool contains(Object element);
 
-  E lookup(E element);
+  E lookup(Object element);
 
   bool remove(Object element);
 
   Iterator<E> get iterator;
 
   Set<E> toSet();
 
   int get length;
 
   bool get isEmpty => length == 0;
@@ skipping 255 matching lines @@
 abstract class SetBase<E> extends SetMixin<E> {
   /**
    * Convert a `Set` to a string as `{each, element, as, string}`.
    *
    * Handles circular references where converting one of the elements
    * to a string ends up converting [set] to a string again.
    */
   static String setToString(Set set) =>
       IterableBase.iterableToFullString(set, '{', '}');
 }