Revert "Remove Expect from core library."

tests/language/const_factory_redirection_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";
-
 // Test that factory redirections work for compile-time constants, and
 // that abstract classes can redirect.
 
 abstract class C {
   const factory C(int x) = D;
 }
 
 class D implements C {
   final int i;
   const D(this.i);
   m() => 'called m';
 }
 
 main() {
   const C c = const C(42);
   D d = c;
   Expect.equals(42, d.i);
   Expect.equals('called m', d.m());
   d = new C(42);
   Expect.equals(42, d.i);
   Expect.equals('called m', d.m());
 }