Revert "Remove Expect from core library."

tests/corelib/reg_exp_start_end_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";
-
 main() {
   var matches =
       new RegExp("(a(b)((c|de)+))").allMatches("abcde abcde abcde");
   var it = matches.iterator;
   int start = 0;
   int end = 5;
   while (it.moveNext()) {
     Match match = it.current;
     Expect.equals(start, match.start);
     Expect.equals(end, match.end);
     start += 6;
     end += 6;
   }
 }