Revert "Remove Expect from core library."

tests/corelib/set_test.dart

 // Copyright (c) 2011, 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 set_test;
 
-
-import 'package:expect/expect.dart';
 import "dart:collection";
 
 void testMain(Set create()) {
   Set set = create();
   Expect.equals(0, set.length);
   set.add(1);
   Expect.equals(1, set.length);
   Expect.equals(true, set.contains(1));
 
   set.add(1);
@@ skipping 147 matching lines @@
   set.clear();
   Expect.equals(0, set.length);
   set.add(11);
   Expect.equals(1, set.length);
 }
 
 main() {
   testMain(() => new Set());
   testMain(() => new HashSet());
 }