Add dummy dart:html library for documentation.

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

Index: lib/html/doc/nodoc-src/_IceCandidateFactoryProvider.dart
diff --git a/lib/html/doc/nodoc-src/_IceCandidateFactoryProvider.dart b/lib/html/doc/nodoc-src/_IceCandidateFactoryProvider.dart
new file mode 100644
index 0000000000000000000000000000000000000000..00d8dc6a263204994ad1c12c13fcfaa183d65d56
--- /dev/null
+++ b/lib/html/doc/nodoc-src/_IceCandidateFactoryProvider.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 _IceCandidateFactoryProvider {
+  factory IceCandidate(String label, String candidateLine) => null;
+}