Revert "Remove Expect from core library."

tests/language/static_const_field_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 static const fields.
 
-import "package:expect/expect.dart";
-
 abstract class Spain {
   static const AG = "Antoni Gaudi";
   static const SD = "Salvador Dali";
 }
 
 abstract class Switzerland {
   static const AG = "Alberto Giacometti";
   static const LC = "Le Corbusier";
 }
 
@@ skipping 29 matching lines @@
     Expect.equals(true, Spain.SD == "Salvador Dali");
     Expect.equals(true, A.artist2 == "Alberto Giacometti");
     Expect.equals(true, A.architect1 == "Antoni Gaudi");
     Expect.equals(2, A.map1["Tuesday"]);
   }
 }
 
 main() {
   StaticFinalFieldTest.testMain();
 }