The answer to your question lies right in the next line in the transpiled code:
else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
To understand it you need to keep in mind these things:
Now, if you read again, you see that "Reflect.decorate" is a method that goes through the list of decorators that you intend to apply, then identifies the type of decorator based on the number of arguments, and finally calls the decorator with the right arguments (based on the type that it detected).
Reflect.decorateif they have it right there?
Because at some point in the future they expect
Reflect.decorate to become part of the ES standard and then be implemented in browsers accordingly. At that moment, they'd rather use the native method than this polyfill.
As this method is part of "ES.later" (not even ES.next), no browser has implemented it natively yet, and for that reason nobody documents it yet.