Revert "Remove Expect from core library."

tests/corelib/date_time2_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";
-
 // Dart test program for DateTime's hashCode.
 
 main() {
   var d = DateTime.parse("2000-01-01T00:00:00Z");
   var d2 = DateTime.parse("2000-01-01T00:00:01Z");
   // There is no guarantee that the hashcode for these two dates is different,
   // but in the worst case we will have to fix this test.
   // The important test here is, that DateTime .
   Expect.isFalse(d.hashCode == d2.hashCode);
 }