createElementUpgrader method
Creates an element upgrader which can be used to change the Dart wrapper type for elements.
The type specified must be a subclass of HtmlElement, when an element is upgraded then the created constructor will be invoked on that element.
If the type is not a direct subclass of HtmlElement then the extendsTag parameter must be provided.
Implementation
ElementUpgrader createElementUpgrader(Type type, {String extendsTag}) {
return new _JSElementUpgrader(this, type, extendsTag);
}