function getSuperClass(constructor) {
  const proto = Object.getPrototypeOf(constructor.prototype);
  return proto && proto.constructor;
}
Given a ES5 or ES6 class reference, return its super class / super
constructor.
This should be used in rare cases where you need to walk up the inheritance
tree (this is generally a bad idea). But this work with ES5 and ES6 classes,
unlike relying on the superClass_ property.
Note: To start walking up the hierarchy from an instance call this with its
`constructor` property; e.g. `getSuperClass(instance.constructor)`.
@param {function(new: ?)} constructor
@return {?Object}