Summary

Adds a yarn version to this repo that will be used instead of local versions of yarn. This will keep things consistent. There are a few steps here:

• add a single-file yarn js bundle (version 1.12.3) to ./yarn-1.12.3.js from yarn/releases (choose 1.12.3 as it was more stable than 1.13.0 which was just released). This is all of yarn (and all of it's dependencies) in a single JS file.
• add yarnrc file with a yarn-path entry that points to the above file
• require yarn 1.0.0 in package.json's engines, as only yarn 1.0.0 will support deferring commands to the yarn-path
• extra credit: remove double dash (--) in yarn in package.json scripts, as this give annoying warnings in yarn 1+ (and now we'll all be on the same version).

How to test

• do not upgrade your yarn version. This should workflow should be doable with any yarn version.
• all of the yarn commands should work: yarn install, yarn test, yarn run some-script...
• run yarn -v. should be 1.12.3
• go out of this directory (e.g. cd ~) and run yarn -v. should be your local version NOT 1.12.3 (unless you just so happen to have this exact version installed locally).

How it works

[email protected] can defer execution to any yarn version included with the code, as called out in .yarnrc.

• you run a yarn command, like yarn install or yarn run some-script
• IF you have yarn >= 1.0.0 it will look for a ./.yarnrc file, and if it finds a yarn-path entry it will defer all yarn commands to that file. In this case it's ./yarn-1.12.3.js.
• ELSE yarn 0.x will just continue on as normal, but...
• whichever yarn version is running will check package.json engines and ensure that it meets the criterion. In this case, >=1.0.0. If it does not, it will fail and bail out. This prevents a user with an old version of yarn from messing up the system.

Why?

Consistency and reproducibility!

This also prevent conflicting yarn features in yarn.lock. For example if developer A has [email protected] it will write integrity checks into the file. If developer B has [email protected], it will constantly remove those features, which means that every install gets tons of unrelated changes to yarn.lock. yarn.lock merge conflicts can be very hard to sort out, so it's best to remove this noise.