Worker: correct usage of importScripts

According to [this document](https://developer.mozilla.org/en-US/docs/Web/Guide/Performance/Using_web_workers#Importing_scripts_and_libraries):

>  Note: Scripts may be downloaded in any order, but will be executed in the order in which you pass the filenames into `importScripts()` .  This is done synchronously; `importScripts()` does not return until all the scripts have been loaded and executed.

This patch fixes our usage of `importScripts` to match the described usage.