Revert "Remove Expect from core library."

tests/corelib/date_time7_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 timeZoneName and timeZoneOffset getters.
 
 testUtc() {
   var d = DateTime.parse("2012-03-04T03:25:38.123Z");
   Expect.equals("UTC", d.timeZoneName);
   Expect.equals(0, d.timeZoneOffset.inSeconds);
 }
 
 testLocal() {
   checkOffset(String name, Duration offset) {
@@ skipping 24 matching lines @@
 
   d = DateTime.parse("2012-07-02T13:45:23");
   name = d.timeZoneName;
   checkOffset(name, d.timeZoneOffset);
 }
 
 main() {
   testUtc();
   testLocal();
 }