function isRequiredPropType(propTypeExpression) {
  return propTypeExpression.type === 'MemberExpression'
    && propTypeExpression.property.name === 'isRequired';
}
Checks if the PropTypes MemberExpression node passed in declares a required propType.
@param {ASTNode} propTypeExpression node to check. Must be a `PropTypes` MemberExpression.
@returns {boolean} `true` if this PropType is required, `false` if not.