Remove stale files from lib/html/src.

lib/html/src/PageTransitionEventWrappingImplementation.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 PageTransitionEventWrappingImplementation extends EventWrappingImplementation implements PageTransitionEvent {
-  PageTransitionEventWrappingImplementation._wrap(ptr) : super._wrap(ptr);
-
-  factory PageTransitionEventWrappingImplementation(String type,
-      [bool canBubble = true, bool cancelable = true,
-      bool persisted = false]) {
-    final e = dom.document.createEvent("PageTransitionEvent");
-    e.initPageTransitionEvent(type, canBubble, cancelable, persisted);
-    return LevelDom.wrapPageTransitionEvent(e);
-  }
-
-  bool get persisted() => _ptr.persisted;
-}