Add dummy dart:html library for documentation.

lib/html/doc/interface/ButtonElement.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 HTMLButtonElement
+interface ButtonElement extends Element default _Elements {
+
+  ButtonElement();
+
+  /** @domName HTMLButtonElement.autofocus */
+  bool autofocus;
+
+  /** @domName HTMLButtonElement.disabled */
+  bool disabled;
+
+  /** @domName HTMLButtonElement.form */
+  final FormElement form;
+
+  /** @domName HTMLButtonElement.formAction */
+  String formAction;
+
+  /** @domName HTMLButtonElement.formEnctype */
+  String formEnctype;
+
+  /** @domName HTMLButtonElement.formMethod */
+  String formMethod;
+
+  /** @domName HTMLButtonElement.formNoValidate */
+  bool formNoValidate;
+
+  /** @domName HTMLButtonElement.formTarget */
+  String formTarget;
+
+  /** @domName HTMLButtonElement.labels */
+  final NodeList labels;
+
+  /** @domName HTMLButtonElement.name */
+  String name;
+
+  /** @domName HTMLButtonElement.type */
+  final String type;
+
+  /** @domName HTMLButtonElement.validationMessage */
+  final String validationMessage;
+
+  /** @domName HTMLButtonElement.validity */
+  final ValidityState validity;
+
+  /** @domName HTMLButtonElement.value */
+  String value;
+
+  /** @domName HTMLButtonElement.willValidate */
+  final bool willValidate;
+
+  /** @domName HTMLButtonElement.checkValidity */
+  bool checkValidity();
+
+  /** @domName HTMLButtonElement.setCustomValidity */
+  void setCustomValidity(String error);
+}