function ecommerce(event, track, arr) {
  push.apply(null, [
    '_fxm.ecommerce.' + event,
    track.id() || track.sku(),
    track.name(),
    track.category()
  ].concat(arr || []));
}
Track ecommerce `event` with `track`
with optional `arr` to append.

@api private
@param {string} event
@param {Track} track
@param {Array} arr