Remove stale files from lib/html/src.

lib/html/src/UIEventWrappingImplementation.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 UIEventWrappingImplementation extends EventWrappingImplementation implements UIEvent {
-  UIEventWrappingImplementation._wrap(ptr) : super._wrap(ptr);
-
-  factory UIEventWrappingImplementation(String type, Window view, int detail,
-      [bool canBubble = true, bool cancelable = true]) {
-    final e = dom.document.createEvent("UIEvent");
-    e.initUIEvent(type, canBubble, cancelable, LevelDom.unwrap(view), detail);
-    return LevelDom.wrapUIEvent(e);
-  }
-
-  int get charCode() => _ptr.charCode;
-
-  int get detail() => _ptr.detail;
-
-  int get keyCode() => _ptr.keyCode;
-
-  int get layerX() => _ptr.layerX;
-
-  int get layerY() => _ptr.layerY;
-
-  int get pageX() => _ptr.pageX;
-
-  int get pageY() => _ptr.pageY;
-
-  Window get view() => LevelDom.wrapWindow(_ptr.view);
-
-  int get which() => _ptr.which;
-}