🛠️ Repo: Compute in-browser test coverage numbers

[boneskull]

We have coverage numbers, but only in Node.js. I'd like to see the coverage numbers against tests run with Karma.

Gathering coverage from Karma seems fraught with peril.

I would entertain other suggestions to improve how we run browser-based tests. My only requirement is that the tests run in a strict browser-like environment--jsdom shenanigans or Electron won't do.

[ScottFreeCode]

I've been working on this in the multi-coverage branch.

Getting Coveralls to take reports from multiple build jobs in the same build turned out to be way easier than I thought. I wasted so many hours looking for a more complex solution because I didn't initially realize I could just apply COVERAGE=true to the whole build. That alone should increase the coverage somewhat as it will show branches taken in different Node versions; would be nice to get more such branches covered though (Windows, whatever other terminals we're handling).

After that, I looked into adding coverage to the browser runs. Here's what I found out.

• karma-coverage can instrument the code and report on coverage for instrumented code, however...
• karma-browserify doesn't look at the results of other preprocessors, therefore...
• browserify-istanbul should be used to make karma-browserify instrument the code, which can then be reported upon by karma-coverage.
• browserify-istanbul mysteriously doesn't run on anything going into the bundle that isn't in Karma's files list and given ['browserify'] on Karma's preprocessors list, so we have to add the relevant patterns to those lists even if they shouldn't be served directly.
• After I got all that figured it, it finally worked! ...Until I tried to clean up the other false starts I'd had, use NYC as described below, and consolidate the code behind if (env.COVERAGE); then something changed and caused the current "bundle error" problems -- but I haven't been able to figure out what.
• It is possible to override the coverage implementation for both browserify-istanbul and karma-coverage; this can be used to make browserify-istanbul use the same version of Istanbul as karma-coverage, or to make both use the same version of Istanbul as our Node CLI NYC. (This seems to work ok except for the above-mentioned issue, and removing it does not seem to resolve the above-mentioned issue.)

Feel free to pull it down locally and experiment to confirm or deny any of the above and try to fix the bundle errors.

[ScottFreeCode]

(I should add that the multi-coverage branch is based off the CI fixes branch, for obvious reasons; the diff from master should get to be simpler once the CI fixes branch is merged, presumably...)

[ScottFreeCode]

Found and fixed what I'd screwed up; that branch is working now (modulo Sauce flakes), so I've submitted PR #2886 for it.