Revert "Remove Expect from core library."

tests/corelib/string_replace_dollar_test.dart

 // Copyright (c) 2012, 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() {
   String jsText = r"""'$'
 """;
   String htmlStr = '%%DART';
   String htmlOut = htmlStr.replaceAll("%%DART", jsText);
   Expect.equals(jsText, htmlOut);
   htmlOut = htmlStr.replaceFirst("%%DART", jsText);
   Expect.equals(jsText, htmlOut);
   htmlOut = htmlStr.replaceAll(new RegExp("%%DART"), jsText);
   Expect.equals(jsText, htmlOut);
   htmlOut = htmlStr.replaceFirst(new RegExp("%%DART"), jsText);
   Expect.equals(jsText, htmlOut);
 }