Move dart:web to pkg:htmlescape.

lib/web/web.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.
-
-#library("dart:web");
-
-// Web related functions and data that are not specific to a single environment
-// (e.g., only in a browser or only on a server).
-
-/**
- * Escapes HTML-special characters of [text] so that the result can be
- * included verbatim in HTML source code, either in an element body or in an
- * attribute value.
- */
-String htmlEscape(String text) {
-  // TODO(efortuna): A more efficient implementation.
-  return text.replaceAll("&", "&")
-             .replaceAll("<", "&lt;")
-             .replaceAll(">", "&gt;")
-             .replaceAll('"', "&quot;")
-             .replaceAll("'", "&apos;");
-}