Revert "Remove Expect from core library."

tests/language/factory_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.
 // Dart test program for testing factories.
 
-import "package:expect/expect.dart";
-
 class A {
   factory A(n) {
     return new A.internal(n);
   }
   A.internal(n) : n_ = n {}
   var n_;
 }
 
 class B {
   factory B.my() {
     return new B(3);
   }
   B(n) : n_ = n {}
   var n_;
 }
 
 class FactoryTest {
   static testMain() {
     new B.my();
     var b = new B.my();
     Expect.equals(3, b.n_);
     var a = new A(5);
     Expect.equals(5, a.n_);
   }
 }
 
 main() {
   FactoryTest.testMain();
 }