Add dummy dart:html library for documentation.

lib/html/doc/interface/TrackElement.dart

Index: lib/html/doc/interface/TrackElement.dart
diff --git a/lib/html/doc/interface/TrackElement.dart b/lib/html/doc/interface/TrackElement.dart
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+++ b/lib/html/doc/interface/TrackElement.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 HTMLTrackElement
+interface TrackElement extends Element default _Elements {
+
+  TrackElement();
+
+  static final int ERROR = 3;
+
+  static final int LOADED = 2;
+
+  static final int LOADING = 1;
+
+  static final int NONE = 0;
+
+  /** @domName HTMLTrackElement.defaultValue */
+  bool defaultValue;
+
+  /** @domName HTMLTrackElement.kind */
+  String kind;
+
+  /** @domName HTMLTrackElement.label */
+  String label;
+
+  /** @domName HTMLTrackElement.readyState */
+  final int readyState;
+
+  /** @domName HTMLTrackElement.src */
+  String src;
+
+  /** @domName HTMLTrackElement.srclang */
+  String srclang;
+
+  /** @domName HTMLTrackElement.track */
+  final TextTrack track;
+}