Revert "Remove Expect from core library."

tests/corelib/reg_exp5_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.
 // Dart test for testing regular expressions in Dart.
 
-import "package:expect/expect.dart";
-
 main() {
   String str = "";
   try {
     RegExp ex = new RegExp(str);
   } catch (e) {
     if (!(e is ArgumentError)) {
       Expect.fail("Expected: ArgumentError got: ${e}");
     }
   }
   Expect.isFalse(new RegExp(r"^\w+$").hasMatch(str));
   Match fm = new RegExp(r"^\w+$").firstMatch(str);
   Expect.equals(null, fm);
 
   Iterable<Match> am = new RegExp(r"^\w+$").allMatches(str);
   Expect.isFalse(am.iterator.moveNext());
 
   Expect.equals(null, new RegExp(r"^\w+$").stringMatch(str));
 }