UIEvent 构造函数
实现
factory UIEvent(String type,
{Window? view,
int detail = 0,
bool canBubble = true,
bool cancelable = true}) {
if (view == null) {
view = window;
}
UIEvent e = document._createEvent("UIEvent") as UIEvent;
e._initUIEvent(type, canBubble, cancelable, view, detail);
return e;
}