feat: make __coverage__ the default approach we advocate for ES2015 coverage

[bcoe]

related to #266

The reason that setting include = / makes __coverage__ compatible with nyc is that / results in no files matching shouldInstrumentFile().

When nyc is configured with a rule that does match shouldInstrumentFile() it results in the source-map information populated by __coverage__ being clobbered, breaking output.

Rather than relying on this side-effect I propose that we do the following:

• add a configuration option that allows source-maps to be turned off in nyc.
• add a noop instrumenter so that we can skip having nyc instrument files (__coverage__ handles this).
• pull nyc's shouldInstrumentFile logic into a module that can be used by both nyc and __coverage__, this way folks can opt to use nyc's include/exclude functionality, while using __coverage__ for instrumentation.
  • https://github.com/bcoe/test-exclude
• update nyc's documentation, documenting the usage of __coverage__ for ES2015 instrumentation.
• direct folks in Better ES2015 Support #266 towards using __coverage__ as a fix for their test-coverage issues.

@jamestalmage @dtinth does this sound reasonable?

[novemberborn]

Hey @bcoe, apologies for the radio silence, I've been traveling 😄

This makes sense, though I wonder if we can shove more behavior into the instrumenter. nyc's default logic could take care of the source maps, for instance. We could also power --all and the writing of the coverage data to disk.

That would allow tools like copenhagen (if I get a chance to work on it) to plug in to nyc more neatly.

[bcoe]

@novemberborn happy to gradually pull more logic into sub-modules, being mindful of the logic that will probably be shared across instrumentation libraries, e.g., in the case of --all, I imagine the iteration logic would remain fairly consistent, but a replacement instrument would be for asking the actual question.

Could you do me a favor and take nyc@next for a spin on a few of your projects, the __coverage__ logic will not be fully functional until @dtinth lands:

dtinth/babel-plugin-__coverage__#35