onMouseDown property Null safety
Dispatched when a user presses the pointing device button over an InteractiveObject instance.
If the target is a SimpleButton instance, the SimpleButton instance displays the downState display object as the default behavior. If the target is a selectable text field, the text field begins selection as the default behavior.
EventStream<MouseEvent> get onMouseDown => InteractiveObject.mouseDownEvent.forTarget(this);