Revert "Remove Expect from core library."

tests/language/prefix5_negative_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.
 
 library Prefix5NegativeTest.dart;
-import "package:expect/expect.dart";
 import "library10.dart";
 
 class Prefix5NegativeTest {
   static Test1() {
     // Library prefixes in the imported libraries should not be visible here.
     var result = 0;
     result += lib11.Library11.static_func();
     Expect.equals(6, result);
     result += lib11.Library11.static_fld;
     Expect.equals(10, result);
   }
 }
 
 main() {
   Prefix5NegativeTest.Test1();
 }