missing extension in import paths in @babel/runtime/helpers/esm

Bug Report

File extensions are required in import statements in ES Modules.

But files within @babel/runtime/helpers/esm (which are treated as ES Modules because @babel/runtime/helpers/esm/package.json contains type: "module") don’t use extensions in their import statements.

• I would like to work on a fix!

Current behavior

I’m getting this correct error with Webpack 5:

ERROR in ./node_modules/@babel/runtime/helpers/esm/get.js 1:0-44
Module not found: Error: Can't resolve './superPropBase' in '/Users/andy/clarity/node_modules/@babel/runtime/helpers/esm'
Did you mean 'superPropBase.js'?
BREAKING CHANGE: The request './superPropBase' failed to resolve only because it was resolved as fully specified
(probably because the origin is a '*.mjs' file or a '*.js' file where the package.json contains '"type": "module"').
The extension in the request is mandatory for it to be fully specified.
Add the extension to the request.

This is because the first line of @babel/runtime/helpers/esm/get.js is an import without an extension:

import superPropBase from "./superPropBase";

Input Code

var your => (code) => here;

Expected behavior

No problems using @babel/plugin-transform-runtime with useESModules: true

Babel Configuration

• Filename: (babel config inlined in babel-loader webpack config)

{
  presets: [
    ['@babel/preset-env', {targets: {browsers: 'last 2 versions'}}],
  ],
  plugins: [
    [
      '@babel/plugin-transform-runtime',
      {
        useESModules: true,
      },
    ],
  ]
}

Environment

• @babel/runtime: 7.11.2
• @babel/plugin-transform-runtime: 7.11.5
• webpack: 5.0.0-rc.4
• babel-loader: 8.1.0

Possible Solution

Use full extensions in imports inside @babel/runtime/helpers/esm, so I don’t get these errors, for example:

import superPropBase from "./superPropBase.js";

[JLHwung]

Duplicate of #12003

[JLHwung]

Fixed in @babel/runtime@7.12.0.