Revert "Remove Expect from core library."

tests/language/closure_with_super_send_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";
-
 // Test a closurized super send.
 
 class Super {
   m() => "super";
 }
 
 class Sub extends Super {
   m() => "sub";
 
   test() {
     var x;
     [0].forEach((e) => x = super.m());
     return x;
   }
 }
 
 main() {
   Expect.equals("super", new Sub().test());
   Expect.equals("super", new Super().m());
   Expect.equals("sub", new Sub().m());
 }