Ensure babel extends builtins on jsnext build

[existentialism]

Was digging into an issue I'm seeing in our app that popped up in redux-form v6.2 (which bumped es6-error to v4).

We use Webpack2 which in turn uses the jsnext build which currently skips babelrc:
https://github.com/bjyoungblood/es6-error/pull/26/files#diff-efd59cd1bdc5c9ac6d0eaa368f1e149fR18

This causes some instanceof breakage since it gets built without babel-plugin-transform-builtin-extend.

[PhilippSpo]

Any updates on this? I also have this issue using redux-form and webpack 2.

[bjyoungblood]

So many bundlers and environments to worry about these days! 😓

[existentialism]

Thanks @bjyoungblood!