Reapply "Add dart:web to dart2js and editor, containing only 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("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;");
+}