Add constructors to all the event classes.

client/html/src/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);
 
-class BeforeLoadEventWrappingImplementation extends EventWrappingImplementation implements BeforeLoadEvent {
-  BeforeLoadEventWrappingImplementation._wrap(ptr) : super._wrap(ptr) {}
+  factory BeforeLoadEventWrappingImplementation(String type, String url,
+      [bool canBubble = true, bool cancelable = true]) {
+    final e = dom.document.createEvent("BeforeLoadEvent");
+    e.initBeforeLoadEvent(type, canBubble, cancelable, url);
+    return LevelDom.wrapBeforeLoadEvent(e);
+  }
 
-  String get url() { return _ptr.url; }
-
-  void initBeforeLoadEvent(String type, bool canBubble, bool cancelable, String url) {
-    _ptr.initBeforeLoadEvent(type, canBubble, cancelable, url);
-    return;
-  }
+  String get url() => _ptr.url;
 }