Revert "Remove Expect from core library."

tests/language/default_factory_library_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.
 
 // Dart test program for testing factories defined across libraries
 
 library test;
-import "package:expect/expect.dart";
 import "default_factory_library.dart" as lib;
 
 class B implements lib.A, C {
   int methodA() { return 1; }
   int methodB() { return 2; }
 }
 
 class C {
   // Referenced from an abstract class in another library
   factory C.A () { return new B(); }
 }
 
 main() {
   var val = new lib.A();
   Expect.equals(true, (val is B));
   Expect.equals(1, val.methodA());
   Expect.equals(2, val.methodB());
 }