mergeFast = function(one: Object, two: Object): Object {
  var ret = {};
  for (var keyOne in one) {
    ret[keyOne] = one[keyOne];
  }
  for (var keyTwo in two) {
    ret[keyTwo] = two[keyTwo];
  }
  return ret;
}
Faster version of `merge` that doesn't check its arguments and
also merges prototype inherited properties.

@param {object} one Any non-null object.
@param {object} two Any non-null object.
@return {object} Merging of two objects, including prototype
inherited properties.