Remove stale files from lib/html/src.

lib/html/src/BeforeLoadEventWrappingImplementation.dart

-// Copyright (c) 2011, 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 BeforeLoadEventWrappingImplementation extends EventWrappingImplementation implements BeforeLoadEvent {
-  BeforeLoadEventWrappingImplementation._wrap(ptr) : super._wrap(ptr);
-
-  factory BeforeLoadEventWrappingImplementation(String type, String url,
-      [bool canBubble = true, bool cancelable = true]) {
-    final e = dom.document.createEvent("BeforeLoadEvent");
-    e.initBeforeLoadEvent(type, canBubble, cancelable, url);
-    return LevelDom.wrapBeforeLoadEvent(e);
-  }
-
-  String get url() => _ptr.url;
-}