function makeMap(str, expectsLowerCase) {
      const map = Object.create(null);
      const list = str.split(',');
      for (let i = 0; i < list.length; i++) {
          map[list[i]] = true;
      }
      return expectsLowerCase ? val => !!map[val.toLowerCase()] : val => !!map[val];
  }
Make a map and return a function for checking if a key
is in that map.
IMPORTANT: all calls of this function must be prefixed with
\/\*#\_\_PURE\_\_\*\/
So that rollup can tree-shake them if necessary.