Remove stale files from lib/html/src.

lib/html/src/MessageEventWrappingImplementation.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 MessageEventWrappingImplementation extends EventWrappingImplementation implements MessageEvent {
-  MessageEventWrappingImplementation._wrap(ptr) : super._wrap(ptr);
-
-  factory MessageEventWrappingImplementation(String type, String data,
-      String origin, String lastEventId, Window source, MessagePort port,
-      [bool canBubble = true, bool cancelable = true]) {
-    final e = dom.document.createEvent("MessageEvent");
-    e.initMessageEvent(type, canBubble, cancelable, data, origin, lastEventId,
-        LevelDom.unwrap(source), LevelDom.unwrap(port));
-    return LevelDom.wrapMessageEvent(e);
-  }
-
-  String get data() => _ptr.data;
-
-  String get lastEventId() => _ptr.lastEventId;
-
-  MessagePort get messagePort() => LevelDom.wrapMessagePort(_ptr.messagePort);
-
-  String get origin() => _ptr.origin;
-
-  Window get source() => LevelDom.wrapWindow(_ptr.source);
-}