Add dummy dart:html library for documentation.

lib/html/doc/interface/FormElement.dartdoc

Index: lib/html/doc/interface/FormElement.dartdoc
diff --git a/lib/html/doc/interface/FormElement.dartdoc b/lib/html/doc/interface/FormElement.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 HTMLFormElement
+interface FormElement extends Element {
+
+  /** @domName HTMLFormElement.acceptCharset */
+  String acceptCharset;
+
+  /** @domName HTMLFormElement.action */
+  String action;
+
+  /** @domName HTMLFormElement.autocomplete */
+  String autocomplete;
+
+  /** @domName HTMLFormElement.encoding */
+  String encoding;
+
+  /** @domName HTMLFormElement.enctype */
+  String enctype;
+
+  /** @domName HTMLFormElement.length */
+  final int length;
+
+  /** @domName HTMLFormElement.method */
+  String method;
+
+  /** @domName HTMLFormElement.name */
+  String name;
+
+  /** @domName HTMLFormElement.noValidate */
+  bool noValidate;
+
+  /** @domName HTMLFormElement.target */
+  String target;
+
+  /** @domName HTMLFormElement.checkValidity */
+  bool checkValidity();
+
+  /** @domName HTMLFormElement.reset */
+  void reset();
+
+  /** @domName HTMLFormElement.submit */
+  void submit();
+}