Revert "Remove Expect from core library."

tests/corelib/set_iterator_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.
 
-import "package:expect/expect.dart";
-
 class FixedHashCode {
   final int _hashCode;
   const FixedHashCode(this._hashCode);
   int get hashCode { return _hashCode; }
 }
 
 class SetIteratorTest {
   static testMain() {
     testSmallSet();
     testLargeSet();
@@ skipping 116 matching lines @@
         Expect.equals(true, identical(x, element));
       }
       Expect.equals(true, foundIt);
     }
   }
 }
 
 main() {
   SetIteratorTest.testMain();
 }