Console error due to missing source map

Now and then I get this error in the JavaScript console:

> Error en el mapeo fuente: TypeError: NetworkError when attempting to fetch resource.
> URL del recurso: moz-extension://50acaf37-b8f3-48d3-99f4-6408e93480ef/browser-polyfill.min.js
> URL del mapa fuente: browser-polyfill.min.js.map

I understand it happens because you bundle [browser-polyfill.min.js](https://github.com/auchenberg/css-reloader/blob/ab538fc4d694eb96ed378410d65e69be4e0f6065/browser-polyfill.min.js) with a link to a source map:

//# sourceMappingURL=browser-polyfill.min.js.map

... but such map is not included.

Start with browser-polyfill.min.js and inspect its sourceMappingURL reference and the extension assets around it. Reproduce the Firefox console warning, then verify that the finished extension no longer requests a missing browser-polyfill.min.js.map file.

Written by the indexing model from the issue text.