Revert "Remove Expect from core library."

tests/corelib/duration_test.dart

 // Copyright (c) 2013, 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";
-
 main() {
   Duration d;
   d = new Duration(days: 1);
   Expect.equals(86400000000, d.inMicroseconds);
   Expect.equals(86400000, d.inMilliseconds);
   Expect.equals(86400, d.inSeconds);
   Expect.equals(1440, d.inMinutes);
   Expect.equals(24, d.inHours);
   Expect.equals(1, d.inDays);
   d = const Duration(hours: 1);
@@ skipping 202 matching lines @@
   d = const Duration(hours: 1999, minutes: 17, seconds: 42);
   Expect.equals("1999:17:42.000000", d.toString());
 
   d = const Duration(days: -1, hours: -3, minutes: -17, seconds: -42,
                      milliseconds: -823, microseconds: -127);
   Expect.equals("-27:17:42.823127", d.toString());
 
   d = const Duration(hours: -1999, minutes: -17, seconds: -42);
   Expect.equals("-1999:17:42.000000", d.toString());
 }