Revert "Remove Expect from core library."

tests/corelib/reg_exp_group_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";
-
 // Dart test program for RegExp.group.
 
 class RegExpGroupTest {
   static testMain() {
     var match = new RegExp("(a(b)((c|de)+))").firstMatch("abcde");
     Expect.equals('abcde', match.group(0));
     Expect.equals('abcde', match.group(1));
     Expect.equals('b', match.group(2));
     Expect.equals('cde', match[3]);
     Expect.equals('de', match[4]);
   }
 }
 
 main() {
   RegExpGroupTest.testMain();
 }