Add dummy dart:html library for documentation.

lib/html/doc/interface/OutputElement.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 HTMLOutputElement
+interface OutputElement extends Element default _Elements {
+
+  OutputElement();
+
+  /** @domName HTMLOutputElement.defaultValue */
+  String defaultValue;
+
+  /** @domName HTMLOutputElement.form */
+  final FormElement form;
+
+  /** @domName HTMLOutputElement.htmlFor */
+  DOMSettableTokenList htmlFor;
+
+  /** @domName HTMLOutputElement.labels */
+  final NodeList labels;
+
+  /** @domName HTMLOutputElement.name */
+  String name;
+
+  /** @domName HTMLOutputElement.type */
+  final String type;
+
+  /** @domName HTMLOutputElement.validationMessage */
+  final String validationMessage;
+
+  /** @domName HTMLOutputElement.validity */
+  final ValidityState validity;
+
+  /** @domName HTMLOutputElement.value */
+  String value;
+
+  /** @domName HTMLOutputElement.willValidate */
+  final bool willValidate;
+
+  /** @domName HTMLOutputElement.checkValidity */
+  bool checkValidity();
+
+  /** @domName HTMLOutputElement.setCustomValidity */
+  void setCustomValidity(String error);
+}