chore: add tests for #420

[bcoe]

@motiz88 here's a test of the happy-path for the new instrumentation logic. I'm going to go ahead and make sure this works on a Windows VM.

[bcoe]

@motiz88 tests run great on v5 on my crumby Windows VM:

Anything you think I'm missing?

[bcoe]

@motiz88 this feels like a step in the right direction, I think it puts you in a position to use this with babel itself?

My concern is it still doesn't solve the common babel-register case:

• allowing nyc to instrument steps on the toes of babel-plugin-istanbul
• without having nyc instrument, the files are never loaded, due to this logic:

  this.walkAllFiles(this.cwd, function (filename) {
    filename = path.resolve(_this.cwd, filename)
    _this.addFile(filename)
    var coverage = coverageFinder()
    var lastCoverage = _this.instrumenter().lastFileCoverage()
    if (lastCoverage) {
      filename = lastCoverage.path
    }
    if (lastCoverage && _this.exclude.shouldInstrument(filename)) {
      coverage[filename] = lastCoverage
    }
  })

any ideas as to how we might be able to delegate to babel-register, without explicitly requiring the file?

[bcoe]

@motiz88 I've done some more digging, things are actually 99% of the way there; the babel-plugin-istanbul plugin populates everything except the magic value for files included via --all; I'm going to dig into why this might be the case.

[bcoe]

@motiz88 of all things, this turned out to be an issue with babel's cache not playing nice with nyc. I've got --all working with babel-register now 👍