eliminate 5082

Finally identify and eliminate the root cause of npm/npm#5082.

currently known facts

1. on the machines available to me, doesn't happen at all
2. happens during tarball creation, so can be triggered with npm pack (and doesn't require a full npm publish)
3. doesn't just happen for index.js
4. requires a pre-existing, shared cache
5. files are being included after being run through fstream-npm and fstream-ignore's exclusion rules

current hypotheses

1. is two race condtions

• the source of #5082 is something that causes files to not be included in the packed tarball
• another one happens even more rarely within the npm cache and is linked to multiple runs of npm operating on the same package simultaneously

2. is occurring in either fstream, fstream-npm, fstream-ignore, node-tar, or npm's lib/utils/tar.js
3. seems to be happening more in Node.js 6.0.0:

• https://twitter.com/SamVerschueren/status/725308337169006597
• https://twitter.com/sindresorhus/status/725351897629077504
• Bisecting says the commit in Node.js core that made this occur more frequently is b488b19eaf

4. is happening when a file that explicitly shouldn't be ignored is nevertheless not making it into the tarball

current approach

1. add all basic scenarios
2. ~~create a stress-testing harness that will run the tests concurrently ~10 or so times~~
3. instrument npm to figure out where the entries are getting dropped

plz help

git clone https://github.com/othiym23/eliminate-5082
cd eliminate-5082
npm it