function getterOrNull(className, property) {
  var ctor = goog.global[className];
  if (!ctor || !ctor.prototype) {
    return null;
  }
  var descriptor = Object.getOwnPropertyDescriptor(ctor.prototype, property);
  return (descriptor && descriptor.get) || null;
}
Shorthand for `Object.getOwnPropertyDescriptor(...).get` to improve
readability during initialization of `Methods`.
@param {string} className
@param {string} property
@return {?Function}