function createMap() {
  return Object.create(null);
}
Creates a new object without a prototype. This object is useful for lookup without having to
guard against prototypically inherited properties via hasOwnProperty.

Related micro-benchmarks:
- http://jsperf.com/object-create2
- http://jsperf.com/proto-map-lookup/2
- http://jsperf.com/for-in-vs-object-keys2

@returns {Object}