code: function superPropertiesClass(constructor) {
     const superCtor = Object.getPrototypeOf(constructor);
  
     // Note, the `PropertiesMixin` class below only refers to the class
     // generated by this call to the mixin; the instanceof test only works
     // because the mixin is deduped and guaranteed only to apply once, hence
     // all constructors in a proto chain will see the same `PropertiesMixin`
     return (superCtor.prototype instanceof PropertiesMixin) ?
       /** @type {PropertiesMixinConstructor} */ (superCtor) : null;
   }
docs: Returns the super class constructor for the given class, if it is an
  instance of the PropertiesMixin.
  
  @param {!PropertiesMixinConstructor} constructor PropertiesMixin constructor
  @return {PropertiesMixinConstructor} Super class constructor