babel-preset-env: `DEBUG` option

Using targets:
{
  "node": "6"
}

Using modules transform: commonjs

Using plugins:
  transform-es2015-destructuring { "node":"6" }
  transform-es2015-for-of { "node":"6" }
  transform-es2015-function-name { "node":"6" }
  transform-exponentiation-operator { "node":"6" }
  transform-async-to-generator { "node":"6" }
  syntax-trailing-function-commas { "node":"6" }

Using polyfills with `entry` option:

[src/in.js] `import 'babel-polyfill'` was not found.
src/in.js -> lib/in.js
