Revert "Remove Expect from core library."

tests/corelib/date_time6_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 DateTime comparison operators.
 
 main() {
   var d = new DateTime.fromMillisecondsSinceEpoch(0, isUtc: true);
   var d2 = new DateTime.fromMillisecondsSinceEpoch(1, isUtc: true);
   Expect.isTrue(d < d2);
   Expect.isTrue(d <= d2);
   Expect.isTrue(d2 > d);
   Expect.isTrue(d2 >= d);
   Expect.isFalse(d2 < d);
   Expect.isFalse(d2 <= d);
   Expect.isFalse(d > d2);
   Expect.isFalse(d >= d2);
 
   d = new DateTime.fromMillisecondsSinceEpoch(-1, isUtc: true);
   d2 = new DateTime.fromMillisecondsSinceEpoch(0, isUtc: true);
   Expect.isTrue(d < d2);
   Expect.isTrue(d <= d2);
   Expect.isTrue(d2 > d);
   Expect.isTrue(d2 >= d);
   Expect.isFalse(d2 < d);
   Expect.isFalse(d2 <= d);
   Expect.isFalse(d > d2);
   Expect.isFalse(d >= d2);
 }