third_party/polymer3/bower_components/polymer/lib/mixins/properties-mixin.js
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;
 }
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;
 }
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 ; }
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
Returns the super class constructor for the given class if it is an instance of the PropertiesMixin .