onDrag property

ElementStream<MouseEvent> onDrag

A stream of drag events fired when this element currently being dragged.

A drag event is added to this stream as soon as the drag begins. A drag event is also added to this stream at intervals while the drag operation is still ongoing.

Other resources

Drag and drop sample based on the tutorial from HTML5Rocks. Drag and drop specification from WHATWG.

Implementation 

ElementStream<MouseEvent> get onDrag => dragEvent.forElement(this);