Remove Expect from core library.

tests/corelib/stopwatch2_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.
 
 // Dart test program for elapsed getters in stopwatch support.
 
+import "package:expect/expect.dart";
+
 main() {
   Stopwatch sw = new Stopwatch()..start();
   while (sw.elapsedMilliseconds < 2) {
     /* do nothing. */
   }
   sw.stop();
   Expect.equals(sw.elapsedMicroseconds, sw.elapsed.inMicroseconds);
   Expect.equals(sw.elapsedMilliseconds, sw.elapsed.inMilliseconds);
 }