code: function isMixed(item) {
    if (!is.object(item)) return false;
    if (!is.string(item.key)) return false;
    if (!has.call(item, 'value')) return false;
    return true;
  }
docs: Determine if item in mapping array is a valid "mixed" type value
  
  Must be an object with properties "key" (of type string)
  and "value" (of any type)
  
  @api private
  @param {*} item
  @return {Boolean}