Before

> dummy@1.0.0 test-cov C:\Users\Moti Z\Documents\GitHub\dummy
> cross-env BABEL_ENV=test npm run build && nyc --all node dist/a && nyc report --reporter text


> dummy@1.0.0 build C:\Users\Moti Z\Documents\GitHub\dummy
> babel src -d dist

src\a.js -> dist\a.js
src\b.js -> dist\b.js
src\c.js -> dist\c.js
b runs
a runs
----------|----------|----------|----------|----------|----------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines |Uncovered Lines |
----------|----------|----------|----------|----------|----------------|
All files |      100 |      100 |      100 |      100 |                |
 a.js     |      100 |      100 |      100 |      100 |                |
 b.js     |      100 |      100 |      100 |      100 |                |
----------|----------|----------|----------|----------|----------------|
----------|----------|----------|----------|----------|----------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines |Uncovered Lines |
----------|----------|----------|----------|----------|----------------|
All files |      100 |      100 |      100 |      100 |                |
 a.js     |      100 |      100 |      100 |      100 |                |
 b.js     |      100 |      100 |      100 |      100 |                |
----------|----------|----------|----------|----------|----------------|

After

> dummy@1.0.0 test-cov C:\Users\Moti Z\Documents\GitHub\dummy
> cross-env BABEL_ENV=test npm run build && nyc --all node dist/a && nyc report --reporter text


> dummy@1.0.0 build C:\Users\Moti Z\Documents\GitHub\dummy
> babel src -d dist

src\a.js -> dist\a.js
src\b.js -> dist\b.js
src\c.js -> dist\c.js
b runs
a runs
----------|----------|----------|----------|----------|----------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines |Uncovered Lines |
----------|----------|----------|----------|----------|----------------|
All files |       50 |        0 |        0 |       50 |                |
 a.js     |      100 |      100 |      100 |      100 |                |
 b.js     |      100 |      100 |      100 |      100 |                |
 c.js     |        0 |        0 |        0 |        0 |            2,4 |
----------|----------|----------|----------|----------|----------------|
----------|----------|----------|----------|----------|----------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines |Uncovered Lines |
----------|----------|----------|----------|----------|----------------|
All files |       50 |        0 |        0 |       50 |                |
 a.js     |      100 |      100 |      100 |      100 |                |
 b.js     |      100 |      100 |      100 |      100 |                |
 c.js     |        0 |        0 |        0 |        0 |            2,4 |
----------|----------|----------|----------|----------|----------------|

Running this yourself

To get the "before" result - clone, run npm install (or yarn), and run npm run test-cov.

To get the "after" result, do all of the above, and additionally:

1. Check out and build: nyc@6f060d9 (PR #420)
2. Run npm link in the nyc repo
3. Run npm link nyc in this repo
4. npm run test-cov