Revert "Remove Expect from core library."

tests/language/closure_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 for closures.
 
-import "package:expect/expect.dart";
-
 class A {
   var field;
   A(this.field) {}
 }
 
 class ClosureTest {
   static testMain() {
     var o = new A(3);
     foo() => o.field++;
     Expect.equals(3, foo());
     Expect.equals(4, o.field);
   }
 }
 
 main() {
   ClosureTest.testMain();
 }