Remove stale files from lib/html/src.

lib/html/src/ErrorEventWrappingImplementation.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 ErrorEventWrappingImplementation extends EventWrappingImplementation implements ErrorEvent {
-  ErrorEventWrappingImplementation._wrap(ptr) : super._wrap(ptr);
-
-  factory ErrorEventWrappingImplementation(String type, String message,
-      String filename, int lineNo, [bool canBubble = true,
-      bool cancelable = true]) {
-    final e = dom.document.createEvent("ErrorEvent");
-    e.initErrorEvent(type, canBubble, cancelable, message, filename, lineNo);
-    return LevelDom.wrapErrorEvent(e);
-  }
-
-  String get filename() => _ptr.filename;
-
-  int get lineno() => _ptr.lineno;
-
-  String get message() => _ptr.message;
-}