Revert "Remove Expect from core library."

tests/language/setter_no_getter_call_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.
 
-import "package:expect/expect.dart";
-
 var topLevelClosure;
 
 /*    /// 01: runtime error
 get topLevel => topLevelClosure;
 */   /// 01: continued
 set topLevel(var value) { }
 
 initialize() {
   print("initializing");
   topLevelClosure = (x) => x * 2;
 }
 
 main() {
   initialize();
   var x = topLevelClosure(2);
   Expect.equals(4, x);
 
   x = topLevel(3);
   Expect.equals(6, x);
 }