Revert "Remove Expect from core library."

tests/language/setter0_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 setting/getting of fields when
 // only getter/setter methods are specified.
 
-import "package:expect/expect.dart";
-
 class First {
   First(int val) : a_ = val { }
   int a_;
 }
 
 class Second extends First {
   static int c;
 
   Second(int val) : super(val) { }
 
@@ skipping 29 matching lines @@
     Second.testStaticMethod();
     Expect.equals(20, Second.c);
     Expect.equals(20, Second.static_d);
   }
 }
 
 
 main() {
   Setter0Test.testMain();
 }