Add dummy dart:html library for documentation.

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

Index: lib/html/doc/nodoc-src/_TextTrackCueFactoryProvider.dart
diff --git a/lib/html/doc/nodoc-src/_TextTrackCueFactoryProvider.dart b/lib/html/doc/nodoc-src/_TextTrackCueFactoryProvider.dart
new file mode 100644
index 0000000000000000000000000000000000000000..e9165caf3b7d72e88b15da21a0e2deac64678a37
--- /dev/null
+++ b/lib/html/doc/nodoc-src/_TextTrackCueFactoryProvider.dart
@@ -0,0 +1,8 @@
+// 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 _TextTrackCueFactoryProvider {
+  factory TextTrackCue(String id, num startTime, num endTime, String text,
+                       [String settings, bool pauseOnExit]) => null;
+}