Hello all,
You can find this issue while migrating a project with a .babelrc file to Webpack 2.
In my case it was a React/ES6 project.

My .babelrc contained:

{
  "presets": ["es2015", "react"]
}

and I had to change it as follows:

{
  "presets": [
    "react",
    [ "es2015", { "modules": false } ]
  ]
}

Webpack 1 wasn’t able to parse ES2015 modules, so Babel would convert them into CommonJS. Now Webpack 2 can parse ES2015 modules, and is able to eliminate dead code based on which modules are used. With [ "es2015", { "modules": false } ] you tell Babel to stop compiling ES2015 modules.

Enjoy!