Add dummy dart:html library for documentation.

lib/html/doc/nodoc-src/_OptionElementFactoryProvider.dart

Index: lib/html/doc/nodoc-src/_OptionElementFactoryProvider.dart
diff --git a/lib/html/doc/nodoc-src/_OptionElementFactoryProvider.dart b/lib/html/doc/nodoc-src/_OptionElementFactoryProvider.dart
new file mode 100644
index 0000000000000000000000000000000000000000..9c016998b2e80f795f9209685b7d9ca2f5645a56
--- /dev/null
+++ b/lib/html/doc/nodoc-src/_OptionElementFactoryProvider.dart
@@ -0,0 +1,9 @@
+// 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.
+
+class _OptionElementFactoryProvider {
+  factory OptionElement([String data = null, String value = null,
+                         bool defaultSelected = null, bool selected = null])
+      => null;
+}