Move dart:web to pkg:htmlescape.

pkg/htmlescape/htmlescape.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).
+/**
+ * This library contains functions to escape strings for safe inclusion
+ * in HTML source.
+ */
+#library("htmlescape");
 
 /**
  * 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;");
 }