Add constructors to all the event classes.

client/html/generated/src/wrapping/_CustomEventWrappingImplementation.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 CustomEventWrappingImplementation extends EventWrappingImplementation implements CustomEvent {
-  CustomEventWrappingImplementation._wrap(ptr) : super._wrap(ptr) {}
-
-  String get detail() { return _ptr.detail; }
-
-  void initCustomEvent(String typeArg, bool canBubbleArg, bool cancelableArg, Object detailArg) {
-    _ptr.initCustomEvent(typeArg, canBubbleArg, cancelableArg, LevelDom.unwrapMaybePrimitive(detailArg));
-    return;
-  }
-}