SOURCE

function makeMap (
  str,
  expectsLowerCase
) {
  var map = Object.create(null);
    console.log(map)
  var list = str.split(',');
  for (var i = 0; i < list.length; i++) {
    map[list[i]] = true;
  }
    console.log(map)
  return expectsLowerCase
    ? function (val) { return map[val.toLowerCase()]; }
    : function (val) { return map[val]; }
}

/**
 * Check if a attribute is a reserved attribute.
 */
var isReservedAttribute = makeMap('key,ref,slot,slot-scope,is');