Event 构造函数

Event(
  1. String type,
  2. {bool canBubble = true,
  3. bool cancelable = true}
)

实现

factory Event(String type, {bool canBubble = true, bool cancelable = true}) {
  return new Event.eventType('Event', type,
      canBubble: canBubble, cancelable: cancelable);
}