Revert "Remove Expect from core library."

tests/language/prefix11_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 Prefix11Test.dart;
-import "package:expect/expect.dart";
 import "library10.dart";
 import "library11.dart" as lib11;
 
 class Prefix11Test {
   static Test1() {
     var result = 0;
     var obj = new Library10(1);
     result = obj.fld;
     Expect.equals(1, result);
     result += obj.func();
@@ skipping 24 matching lines @@
     Expect.equals(200, lib11.top_level_func11());
   }
 }
 
 main() {
   Prefix11Test.Test1();
   Prefix11Test.Test2();
   Prefix11Test.Test3();
   Prefix11Test.Test4();
 }