Revert "Remove Expect from core library."

tests/corelib/list_iterators_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.
 
-import "package:expect/expect.dart";
-
 class ListIteratorsTest {
   static void checkListIterator(List a) {
     Iterator it = a.iterator;
     Expect.isNull(it.current);
     for (int i = 0; i < a.length; i++) {
       Expect.isTrue(it.moveNext());
       var elem = it.current;
       Expect.equals(a[i], elem);
     }
     Expect.isFalse(it.moveNext());
@@ skipping 24 matching lines @@
     Expect.equals(3, it.current);
     Expect.throws(it.moveNext, (e) => e is ConcurrentModificationError);
     // No progress when throwing.
     Expect.equals(3, it.current);
   }
 }
 
 main() {
   ListIteratorsTest.testMain();
 }