Equivalent of `typeof` but with special handling for array and regexp.
function getPropType(propValue) {
  const propType = typeof propValue;
  if (Array.isArray(propValue)) {
    return 'array';
  }
  if (propValue instanceof RegExp) {
    // Old webkits (at least until Android 4.0) return 'function' rather than
    // 'object' for typeof a RegExp. We'll normalize this here so that /bla/
    // passes PropTypes.object.
    return 'object';
  }
  return propType;
}