In #368, I changed regenerator-transform to remove bindings which are hoisted, since it caused problems when they were registered again.
Nothing in this plugin actually registered them, but sometimes Babel re-traverses the scope and registers declaration which weren't already registerd (this usually happens when a function node is discarded and re-created, I think). For this reason, this code used to throw:

async function foo() {
	(async function (number) {
		const tmp = number
	})
}

because when Babel did re-crawl the bindings, it already had a tmp binding registered (for const tmp) and tried to register a new one with the same name (for the hoisted var tmp).

Unfortunately we can't always rely on something to trigger the scope update, so simply deleting the old binding isn't enough. We also need to ensure that the new hoisted vars get registered again. Otherwise, we cause conflicts with the transform-modules-commonjs plugin (or any other plugin relying on scope information):

import { someAction } from 'actions';

function* foo() {
  const someAction = bar;
}

This code is initially transformed by regenerator to something like this:

import { someAction } from 'actions';

function foo() {
  var someAction;

  return regeneratorRuntime.wrap(function foo() {
    someAction = bar;
  });
}

it correctly removes the const someAction binding from the scope, but before this PR it didn't register the var someAction binding. For this reason, the modules transform plugin thought that someAction = bar was assigning to the imported binding (which is read-only), generating an error.

Fixes babel/babel#10193

PS: Do you have a custom .npmrc for this project on your machine? Every time I run npm test it generates package-lock.json files for the subpackages, and it changes the dependencies section in the top-level package.json file to reference the local packages.