Remove stale files from lib/html/src.

lib/html/src/CloseEventWrappingImplementation.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 CloseEventWrappingImplementation extends EventWrappingImplementation implements CloseEvent {
-  CloseEventWrappingImplementation._wrap(ptr) : super._wrap(ptr);
-
-  factory CloseEventWrappingImplementation(String type, int code, String reason,
-      [bool canBubble = true, bool cancelable = true, bool wasClean = true]) {
-    final e = dom.document.createEvent("CloseEvent");
-    e.initCloseEvent(type, canBubble, cancelable, wasClean, code, reason);
-    return LevelDom.wrapCloseEvent(e);
-  }
-
-  int get code() => _ptr.code;
-
-  String get reason() => _ptr.reason;
-
-  bool get wasClean() => _ptr.wasClean;
-}