Revert "Remove Expect from core library."

tests/compiler/dart2js/static_closure_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.
 
 // Test that static functions are closurized as expected.
 
-import "package:expect/expect.dart";
 import 'compiler_helper.dart';
 
 main() {
   String code = compileAll(r'''main() { print(main); }''');
 
   // At some point, we will have to closurize global functions
   // differently, at which point this test will break. Then it is time
   // to implement a way to call a Dart closure from JS foreign
   // functions.
 
   // If this test fail, please take a look at the use of
   // toStringWrapper in captureStackTrace in js_helper.dart.
   Expect.isTrue(code.contains(r'print($.main);'));
 }