Revert "Remove Expect from core library."

tests/language/const_escape_frog_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.
-// Test division by power of two.
-// Test that results before and after optimization are the same.
-
-import "package:expect/expect.dart";
-
 class Foo {
   const Bar<Foo> bar = const Bar/* comment here use to trigger bug 323 */();
 }
 
 class Bar<T extends Foo> {
   const Bar();
 }
 
 main() {
   Expect.equals(new Foo().bar, const Bar());
 }