Add constructors to all the event classes.

client/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;
+}