Revert "Remove Expect from core library."

tests/language/prefix12_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 Prefix12Test.dart;
-import "package:expect/expect.dart";
 import "library11.dart" as lib11;
 
 class Prefix12Test {
   static Test1() {
     var result = 0;
     var obj = new lib11.Library11.namedConstructor(10);
     result = obj.fld;
     Expect.equals(10, result);
   }
   static Test2() {
     int result = 0;
     var obj = new lib11.Library111<int>.namedConstructor(10);
     result = obj.fld;
     Expect.equals(10, result);
   }
 }
 
 main() {
   Prefix12Test.Test1();
   Prefix12Test.Test2();
 }