Add dummy dart:html library for documentation.

lib/html/doc/interface/OptionElement.dartdoc

Index: lib/html/doc/interface/OptionElement.dartdoc
diff --git a/lib/html/doc/interface/OptionElement.dartdoc b/lib/html/doc/interface/OptionElement.dartdoc
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+// 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.
+
+// WARNING:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @domName HTMLOptionElement
+interface OptionElement extends Element default _OptionElementFactoryProvider {
+
+  OptionElement([String data, String value, bool defaultSelected, bool selected]);
+
+  /** @domName HTMLOptionElement.defaultSelected */
+  bool defaultSelected;
+
+  /** @domName HTMLOptionElement.disabled */
+  bool disabled;
+
+  /** @domName HTMLOptionElement.form */
+  final FormElement form;
+
+  /** @domName HTMLOptionElement.index */
+  final int index;
+
+  /** @domName HTMLOptionElement.label */
+  String label;
+
+  /** @domName HTMLOptionElement.selected */
+  bool selected;
+
+  /** @domName HTMLOptionElement.value */
+  String value;
+}