Revert "Remove Expect from core library."

tests/language/closure_in_initializer2_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";
-
 class S {
   S() {
     Expect.equals(2, this.f());
   }
 }
 
 class A extends S {
   var f;
   A(a) : f = (() => ++a) {
     Expect.equals(a, 2);
   }
 }
 
 main() {
   var a = new A(1);
   Expect.equals(a.f(), 3);
 }