Tasks to Cover (🥁)

• Determine list/filtering of files to instrument for coverage.
• Actually instrument those files. Importantly, using a recent version of istanbul-lib-instrument and not the one that karma-coverage depends on.
• Preserve source maps so that coverage is mapped back to source files.

Implementation Notes

• The additions in packages/angular/build/src/tools/babel/plugins/* are 1:1 copies of files already present in packages/angular_devkit/build_angular/src/tools/babel/plugins/*. I couldn't find a clean way to prevent that. Either @angular/build have to expose the plugin in package.json#exports or it would have to depend on @angular-devkit/build-angular. Both seemed undesirable.
• The logic for excluding files from coverage is lifted from the equivalent webpack config. Given the state of Karma, I didn't make an effort to refactor the code to share the logic - I assume we don't expect many changes here. That would definitely be possible with limited effort though.

Historical Notes

Approaches that didn't work:

• Instrument entire output chunks. karma-coverage really doesn't want to filter data from coverage data after the fact. Not to mention that it could be fairly expensive to send that data back to the karma process if it's then just discarded.
• Implied above: Use karma-coverage's built-in preprocessor. The outdated instrumentation code it's using doesn't handle async function correctly and gives bad function coverage data because of it.

Things that could've worked:

• Apply source maps while instrumenting instead of while post-processing the coverage data. But that would've been swimming against the stream.