Revert "Remove Expect from core library."

tests/corelib/stopwatch_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.
 
 // Dart test program for testing stopwatch support.
 
 library stopwatch_test;
-import "package:expect/expect.dart";
 
 class StopwatchTest {
   static bool checkTicking(Stopwatch sw) {
     Expect.isFalse(sw.isRunning);
     sw.start();
     Expect.isTrue(sw.isRunning);
     for (int i = 0; i < 1000000; i++) {
       int.parse(i.toString());
       if (sw.elapsedTicks > 0) {
         break;
@@ skipping 99 matching lines @@
     Expect.isTrue(checkTicking(sw));
     Expect.isTrue(checkStopping(sw));
     checkRestart();
     checkReset();
   }
 }
 
 main() {
   StopwatchTest.testMain();
 }