Remove stale files from lib/html/src.

lib/html/src/TransitionEventWrappingImplementation.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 TransitionEventWrappingImplementation extends EventWrappingImplementation implements TransitionEvent {
-  static String _name;
-
-  TransitionEventWrappingImplementation._wrap(ptr) : super._wrap(ptr);
-
-  static String get _eventName() {
-    if (_name != null) return _name;
-
-    try {
-      dom.document.createEvent("WebKitTransitionEvent");
-      _name = "WebKitTransitionEvent";
-    } catch (var e) {
-      _name = "TransitionEvent";
-    }
-    return _name;
-  }
-
-  factory TransitionEventWrappingImplementation(String type,
-      String propertyName, double elapsedTime, [bool canBubble = true,
-      bool cancelable = true]) {
-    final e = dom.document.createEvent(_eventName);
-    e.initWebKitTransitionEvent(type, canBubble, cancelable, propertyName,
-        elapsedTime);
-    return LevelDom.wrapTransitionEvent(e);
-  }
-
-  num get elapsedTime() => _ptr.elapsedTime;
-
-  String get propertyName() => _ptr.propertyName;
-}