Add dummy dart:html library for documentation.

lib/html/doc/interface/VideoElement.dart

Index: lib/html/doc/interface/VideoElement.dart
diff --git a/lib/html/doc/interface/VideoElement.dart b/lib/html/doc/interface/VideoElement.dart
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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: Do not edit - generated code.
+
+/// @domName HTMLVideoElement
+interface VideoElement extends MediaElement default _Elements {
+
+  VideoElement();
+
+  /** @domName HTMLVideoElement.height */
+  int height;
+
+  /** @domName HTMLVideoElement.poster */
+  String poster;
+
+  /** @domName HTMLVideoElement.videoHeight */
+  final int videoHeight;
+
+  /** @domName HTMLVideoElement.videoWidth */
+  final int videoWidth;
+
+  /** @domName HTMLVideoElement.webkitDecodedFrameCount */
+  final int webkitDecodedFrameCount;
+
+  /** @domName HTMLVideoElement.webkitDisplayingFullscreen */
+  final bool webkitDisplayingFullscreen;
+
+  /** @domName HTMLVideoElement.webkitDroppedFrameCount */
+  final int webkitDroppedFrameCount;
+
+  /** @domName HTMLVideoElement.webkitSupportsFullscreen */
+  final bool webkitSupportsFullscreen;
+
+  /** @domName HTMLVideoElement.width */
+  int width;
+
+  /** @domName HTMLVideoElement.webkitEnterFullScreen */
+  void webkitEnterFullScreen();
+
+  /** @domName HTMLVideoElement.webkitEnterFullscreen */
+  void webkitEnterFullscreen();
+
+  /** @domName HTMLVideoElement.webkitExitFullScreen */
+  void webkitExitFullScreen();
+
+  /** @domName HTMLVideoElement.webkitExitFullscreen */
+  void webkitExitFullscreen();
+}