Remove explicit interface versions of dartdoc files.

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

-// 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 _AudioContextFactoryProvider {
-
-  factory AudioContext() => null;
-}
-
-class _PointFactoryProvider {
-
-  factory Point(num x, num y) => null;
-}
-
-class _WebSocketFactoryProvider {
-
-  factory WebSocket(String url) => null;
-}
-
-class _TextFactoryProvider {
-  factory Text(String data) => null;
-}