Revert "Remove Expect from core library."

tests/language/reg_exp_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 RegExpTest {
   static test1() {
     RegExp exp = new RegExp("(\\w+)");
     String str = "Parse my string";
     List<Match> matches = new List<Match>.from(exp.allMatches(str));
     Expect.equals(3, matches.length);
     Expect.equals("Parse", matches[0].group(0));
     Expect.equals("my", matches[1].group(0));
     Expect.equals("string", matches[2].group(0));
   }
 
   static testMain() {
     test1();
   }
 }
 
 main() {
   RegExpTest.testMain();
 }