Add dummy dart:html library for documentation.

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

Index: lib/html/doc/nodoc-src/_SessionDescriptionFactoryProvider.dart
diff --git a/lib/html/doc/nodoc-src/_SessionDescriptionFactoryProvider.dart b/lib/html/doc/nodoc-src/_SessionDescriptionFactoryProvider.dart
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index 0000000000000000000000000000000000000000..554708041519cad2741f8c9f7d7cfb90df9dd73e
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+++ b/lib/html/doc/nodoc-src/_SessionDescriptionFactoryProvider.dart
@@ -0,0 +1,7 @@
+// 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 _SessionDescriptionFactoryProvider {
+  factory SessionDescription(String sdp) => null;
+}