Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/operator_equals_test.dart

 // Copyright (c) 2012, 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 AlwaysTrue {
   operator ==(a) => true;
 }
 
 class AlwaysFalse {
   operator ==(b) => false;
 }
 
 main() {
   var a = new AlwaysTrue();
   Expect.isTrue(a == 2);
   Expect.isFalse(a == null);
   Expect.isFalse(a != 2);
   Expect.isTrue(a != null);
 
   a = new AlwaysFalse();
   Expect.isFalse(a == 2);
   Expect.isFalse(a == null);
   Expect.isTrue(a != 2);
   Expect.isTrue(a != null);
 }