Revert "Remove Expect from core library."

tests/isolate/timer_not_available_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.
 
 library timerNotAvailable;
 
-import "package:expect/expect.dart";
 import 'dart:async';
 
 main() {
   final ms = const Duration(milliseconds: 1);
   bool failed = false;
   try {
     new Timer(ms * 5, () { });
   } on UnsupportedError catch (e) {
     failed = true;
   }
   Expect.isTrue(failed);
   failed = false;
   try {
     var t = new Timer.repeating(ms * 10, (_) { });
     t.cancel();
   } on UnsupportedError catch (e) {
     failed = true;
   }
   Expect.isTrue(failed);
 }