Add dummy dart:html library for documentation.

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

Index: lib/html/doc/nodoc-src/_DeprecatedPeerConnectionFactoryProvider.dart
diff --git a/lib/html/doc/nodoc-src/_DeprecatedPeerConnectionFactoryProvider.dart b/lib/html/doc/nodoc-src/_DeprecatedPeerConnectionFactoryProvider.dart
new file mode 100644
index 0000000000000000000000000000000000000000..d17cd5a7a7d2a81bc7b8e9c96795a05ebfaa5a85
--- /dev/null
+++ b/lib/html/doc/nodoc-src/_DeprecatedPeerConnectionFactoryProvider.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 _DeprecatedPeerConnectionFactoryProvider {
+  factory DeprecatedPeerConnection(String serverConfiguration, SignalingCallback signalingCallback) => null;
+}