Errata

babel-cli v6 installs as described, but babel-preset-env is no longer available with the transition to v7.

I had to change the packages from printed:
npm install babel-cli@6 --save-dev
npm install babel-preset-env@6 --save-dev

to this:
npm install @babel/core --save-dev
npm install @babel/cli --save-dev
npm install @babel/preset-env --save-dev

...in order to get the babel-cli installed and transpiling the first example.js successfully.

const validations = new Map()
const validator = {
set(target, key, value) {
if (validations.has(key)) {
return validations[key](value)
}
return Reflect.set(target, key, value)
}
}

validations is an es6 Map so return validations[key](value) is an error.
It should be return validations.get(key)(value)