Bug Report

• I would like to work on a fix!

Current Behavior
I'm using rollup to bundle a package inside a yarn monorepo. As soon as @babel/plugin-transform-runtime is used, the built package breaks because of broken resolved paths to runtime helpers such as _typeof. This results in warnings like this:

(!) Unresolved dependencies

C:UsersjohanneszProjectsMyproject
untime/helpers/typeof (imported by ..\..\..\node_modules\regenerator-runtime\runtime.js,  C:UsersjohanneszProjectsMyproject
untime/helpers/typeof?commonjs-external

As you can see, the Backslashes are missing from the paths, causing a broken build.

See rollup/plugins#290 and yarnpkg/yarn#8031

Input Code

Here is a repro: https://github.com/johannes-z/rollup-yarn-path-error

Expected behavior/code
The paths should be correctly resolved on Windows

Babel Configuration (babel.config.js, .babelrc, package.json#babel, cli command, .eslintrc)

• Filename: babel.config.js

module.exports = {
  presets: [
    [
      '@vue/cli-plugin-babel/preset',
      {
        corejs: 3,
        useBuiltIns: 'usage',
      },
    ],
  ],
}

Environment

  System:
    OS: Windows 10 10.0.18363
  Binaries:
    Node: 11.13.0 - C:\Users\johannesz\scoop\apps\nvm\current\nodejs\nodejs\node.EXE
    Yarn: 1.22.0 - C:\Users\johannesz\scoop\apps\yarn\current\Yarn\bin\yarn.CMD    
    npm: 6.7.0 - C:\Users\johannesz\scoop\apps\nvm\current\nodejs\nodejs\npm.CMD   
  Monorepos:
    Yarn Workspaces: 1.22.0
  npmPackages:
    eslint: ^5.16.0 => 5.16.0
    rollup-plugin-babel: ^4.4.0 => 4.4.0

• How you are using Babel: loader/rollup-plugin-babel

Possible Solution
I was able to fix this problem by fixing the backslashes:
https://github.com/babel/babel/blob/master/packages/babel-plugin-transform-runtime/src/index.js#L237

This is the line where the error ultimately occurs (I don't know where it comes from though).

Changing this line to fixed it for me:

return this.addDefaultImport(
  `${modulePath.replace(/\\/g, '\\\\')}/${helpersDir}/${name}`,
  name,
  blockHoist
);

Of course, this is not a permanent fix; especially since it seems to work on UNIX systems. I think resolveAbsoluteRuntime should consider Windows as an environment and generate a correct path.

Changing it to the following would probably be a better solution to fix this issue:

`${modulePath.replace(/\\/g, '/')}/${helpersDir}/${name}`

EDIT: I investigated a little further. In fact the resolveAbsoluteRuntime function causes inconsistent paths between OS: https://github.com/babel/babel/blob/master/packages/babel-plugin-transform-runtime/src/index.js#L13-L15

path.dirname creates forward slashes for unix systems, but on windows it only creates single backslashes: C:\Users\Johannes\.... Backslashes returned from dirname should be replaced with forward slashes - I haven't found a node.js way of doing this automatically.