Revert "Remove Expect from core library."

tests/compiler/dart2js_native/native_class_inheritance4_frog_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.
 
-import "package:expect/expect.dart";
-
 // Additional Dart code may be 'placed on' hidden native classes.  With
 // inheritance, the superclass method must not be reached by a call on the
 // subclass.
 
 class A native "*A" {
   var _field;
 
   int get X => _field;
   void set X(int x) { _field = x; }
 
@@ skipping 121 matching lines @@
 }
 
 main() {
   testBasicA_dynamic();
   testBasicA_typed();
   testBasicB_dynamic();
   testBasicB_typed();
   testAB_dynamic();
   testAB_typed();
 }