* Migrate core/contextmenu_registry.js to ES6 const/let
* Migrate core/contextmenu_registry.js to goog.module
* Migrate core/contextmenu_registry.js to named requires
* clang-format core/contextmenu_registry.js
* Make core/contextmenu_registry.js constructor private
A {} has a bunch of names already defined on it (like ‘toString’). When using an object as a map with arbitrary keys, it should not inherit from Object.prototype.