Revert "Remove Expect from core library."

tests/language/reg_exp3_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";
-
 class RegExp3Test {
   static testMain() {
     var i = 2000;
     try {
       RegExp exp = new RegExp("[");
       i = 100;  // Should not reach here.
     } on FormatException catch (e) {
       i = 0;
     }
     Expect.equals(0, i);
   }
 }
 
 main() {
   RegExp3Test.testMain();
 }