function isComponentOfType(classType, reactElement) {
  return customChecker
    ? customChecker(classType, reactElement)
    : defaultChecker(classType, reactElement);
}
Executes customChecker if it's set or defaultChecker.

@param classType {ReactElement class} - the class of a React Element
@param reactElement {ReactElement} - any React Element (not a real DOM node)