Add constructors to all the event classes.

client/html/generated/src/wrapping/_BeforeLoadEventWrappingImplementation.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.
-
-// WARNING: Do not edit - generated code.
-
-class BeforeLoadEventWrappingImplementation extends EventWrappingImplementation implements BeforeLoadEvent {
-  BeforeLoadEventWrappingImplementation._wrap(ptr) : super._wrap(ptr) {}
-
-  String get url() { return _ptr.url; }
-
-  void initBeforeLoadEvent(String type, bool canBubble, bool cancelable, String url) {
-    _ptr.initBeforeLoadEvent(type, canBubble, cancelable, url);
-    return;
-  }
-}