function _onKeyDown( event, forceConsider = false ) {
  if ( forceConsider || _shouldConsider( event ) ) {
    const { fn, instance } = store.findBindingForEvent( event ) || {};
    if ( fn ) {
      fn.call( instance, event );
      return true;
    }
  }
  return false;
}
_onKeyDown: The keydown event callback

@access private
@param {object} event The keydown event object
@param {number} event.which The key code (which) received from the keydown event