Fix issues with isoworker

[101arrowz]

I'm not sure what tooling to install to test this locally, so I'm just creating this PR to show the most appropriate usage of isoworker. I don't know how to set up Karma with Mocha, but the build looks fine after bundling, and if it fails that's fixable via https://github.com/reactioncommerce/reaction-component-library/issues/399#issuecomment-467860022.

As a side note, you may want to consider hiding isoworker behind a feature flag that adds local dependency support, since that will reduce bundle size by quite a bit. isoworker, in the name of supporting everything that could possibly need to be serialized, adds around 3kB gzipped to the overall bundle size.

[alewin]

thanks @101arrowz! as soon as I find some free time, I'll check and test the pr