Add dummy dart:html library for documentation.

lib/html/doc/interface/KeygenElement.dartdoc

+// 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 HTMLKeygenElement
+interface KeygenElement extends Element default _Elements {
+
+  KeygenElement();
+
+  /** @domName HTMLKeygenElement.autofocus */
+  bool autofocus;
+
+  /** @domName HTMLKeygenElement.challenge */
+  String challenge;
+
+  /** @domName HTMLKeygenElement.disabled */
+  bool disabled;
+
+  /** @domName HTMLKeygenElement.form */
+  final FormElement form;
+
+  /** @domName HTMLKeygenElement.keytype */
+  String keytype;
+
+  /** @domName HTMLKeygenElement.labels */
+  final NodeList labels;
+
+  /** @domName HTMLKeygenElement.name */
+  String name;
+
+  /** @domName HTMLKeygenElement.type */
+  final String type;
+
+  /** @domName HTMLKeygenElement.validationMessage */
+  final String validationMessage;
+
+  /** @domName HTMLKeygenElement.validity */
+  final ValidityState validity;
+
+  /** @domName HTMLKeygenElement.willValidate */
+  final bool willValidate;
+
+  /** @domName HTMLKeygenElement.checkValidity */
+  bool checkValidity();
+
+  /** @domName HTMLKeygenElement.setCustomValidity */
+  void setCustomValidity(String error);
+}