Add dummy dart:html library for documentation.

lib/html/doc/nodoc-src/_Elements.dart

+// 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.
+
+class _Elements {
+  factory AnchorElement([String href]) => null;
+  factory AreaElement() => null;
+  factory BRElement() => null;
+  factory BaseElement() => null;
+  factory BodyElement() => null;
+  factory ButtonElement() => null;
+  factory CanvasElement([int height, int width]) => null;
+  factory DListElement() => null;
+  factory DetailsElement() => null;
+  factory DivElement() => null;
+  factory EmbedElement() => null;
+  factory FieldSetElement() => null;
+  factory HRElement() => null;
+  factory HeadElement() => null;
+  factory HeadingElement.h1() => null;
+  factory HeadingElement.h2() => null;
+  factory HeadingElement.h3() => null;
+  factory HeadingElement.h4() => null;
+  factory HeadingElement.h5() => null;
+  factory HeadingElement.h6() => null;
+  factory HtmlElement() => null;
+  factory IFrameElement() => null;
+  factory ImageElement([String src, int height, int width]) => null;
+  factory InputElement([String type]) => null;
+  factory KeygenElement() => null;
+  factory LIElement() => null;
+  factory LabelElement() => null;
+  factory LegendElement() => null;
+  factory LinkElement() => null;
+  factory MapElement() => null;
+  factory MenuElement() => null;
+  factory MeterElement() => null;
+  factory OListElement() => null;
+  factory ObjectElement() => null;
+  factory OptGroupElement() => null;
+  factory OutputElement() => null;
+  factory ParagraphElement() => null;
+  factory ParamElement() => null;
+  factory PreElement() => null;
+  factory ProgressElement() => null;
+  factory ScriptElement() => null;
+  factory SourceElement() => null;
+  factory SpanElement() => null;
+  factory StyleElement() => null;
+  factory TableCaptionElement() => null;
+  factory TableCellElement() => null;
+  factory TableColElement() => null;
+  factory TableElement() => null;
+  factory TableRowElement() => null;
+  factory TextAreaElement() => null;
+  factory TitleElement() => null;
+  factory TrackElement() => null;
+  factory UListElement() => null;
+  factory VideoElement() => null;
+}