Adding a method to Function.prototype makes class static calls K.helper() throw 'not a function' #11492

Found while working on #11491 (dispatcher Function.prototype cache). It reproduces on main.

After a user method is added to Function.prototype (for example Function.prototype.myHelper = function(){...}), calling a static method on a class, K.helper(), throws "not a function". Node calls the static method normally.

Needs a package-free repro and a fix in class static dispatch or own-property lookup on class constructors.