Convert ol.CollectionEvent#getElement method into element property
Convert ol.interaction.DragAndDropEvent#get* methods into properties
Convert ol.ObjectEvent#getKey method into key property
Convert ol.render.Event#get* methods into properties
Convert ol.source.VectorEvent#getFeature method into feature property
Convert ol.DrawEvent#getFeature method into feature property
Convert ol.MapBrowserEvent#get* methods into properties
Merge pull request #1588 from twpayne/event-properties

Use properties, not get* methods, in events