Revert "Remove Expect from core library."

tests/language/field_increment_bailout_test.dart

 // Copyright (c) 2013, 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.
 
 // dart2js regression test for issue 8781.
 
-import "package:expect/expect.dart";
-
 class N {
   var outgoing;
   var incoming;
   N(this.outgoing, this.incoming);
 }
 
 class A {
   int offset = 0;
   var list;
   var node;
@@ skipping 23 matching lines @@
 }
 
 main () {
   var o = new A(new N(new L([1]), new L([2])));
 
   for (var i = 1; i <= 2; i++)
     Expect.equals(i, o.next());
 
   Expect.equals(null, o.list);
 }