Remove stale files from lib/html/src.

lib/html/src/CustomEventWrappingImplementation.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 CustomEventWrappingImplementation extends EventWrappingImplementation implements CustomEvent {
-  CustomEventWrappingImplementation._wrap(ptr) : super._wrap(ptr);
-
-  factory CustomEventWrappingImplementation(String type,
-      [bool canBubble = true, bool cancelable = true, Object detail = null]) {
-    final e = dom.document.createEvent("CustomEvent");
-    e.initCustomEvent(type, canBubble, cancelable, detail);
-    return LevelDom.wrapCustomEvent(e);
-  }
-
-  String get detail() => _ptr.detail;
-}