Revert "Remove Expect from core library."

tests/corelib/list_growable_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.
 
-import "package:expect/expect.dart";
-
 main() {
   var a;
   a = new List();
   a.add(499);
   Expect.equals(1, a.length);
   Expect.equals(499, a[0]);
   a.clear();
   Expect.equals(0, a.length);
   Expect.throws(() => a[0], (e) => e is RangeError);
 
@@ skipping 12 matching lines @@
   a.add(499);
   Expect.equals(43, a.length);
   Expect.equals(499, a[42]);
   for (int i = 0; i < 42; i++) {
     Expect.equals(null, a[i]);
   }
   a.clear();
   Expect.equals(0, a.length);
   Expect.throws(() => a[0], (e) => e is RangeError);
 }