Fix Istanbul reporting different data.

[itsjamie]

Is your feature request related to a problem? Please describe.
Istanbul is reporting odd behaviour where the number of run tests can change between executions and thus the coverage can change quite a bit run to run.

Describe the solution you'd like
I wonder if it is because of the out of date Webpack loader we're using to work with Istanbul. It seems like some of the community has forked from https://github.com/webpack-contrib/istanbul-instrumenter-loader to https://github.com/JS-DevTools/coverage-istanbul-loader instead because of lingering PRs. I wonder if updating to use this new package would help.

Describe alternatives you've considered
I wondered if we were bringing all the testing sources in properly, haven't confirmed either way.

Additional context
#2123 (comment)

[itsjamie]

@johnBartos you weren't crazy.

Looking into updating and seeing if it can still be reproduced.

[itsjamie]

Tried upgrading Karma, changing Istanbul coverage reporter, and switching back to bundling a webpack build for every test rather than a single entry test.

All of these methods still had shaky reporting.