Revert "Remove Expect from core library."

tests/language/ackermann_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.
 // Dart version of two-argument Ackermann-Peter function.
 
-import "package:expect/expect.dart";
-
 class AckermannTest {
   static ack(m, n) {
     return m == 0 ?
         n + 1 : ((n == 0) ?
                     ack(m - 1, 1) : ack(m - 1, ack(m, n - 1)));
   }
 
   static testMain() {
     Expect.equals(253, ack(3, 5));
   }
 }
 
 main() {
   AckermannTest.testMain();
 }