code: function mousedown(inEvent) {
    if (!this.isEventSimulatedFromTouch_(inEvent)) {
      // TODO(dfreedman) workaround for some elements not sending mouseup
      // http://crbug/149091
      if (POINTER_ID.toString() in this.pointerMap) {
        this.cancel(inEvent);
      }
      const e = prepareEvent(inEvent, this.dispatcher);
      this.pointerMap[POINTER_ID.toString()] = inEvent;
      this.dispatcher.down(e, inEvent);
    }
  }
docs: Handler for `mousedown`.
  
  @this {MouseSource}
  @param {MouseEvent} inEvent The in event.