Re-enable ES6 modules via Webpack when they stop breaking sourcemaps

[gaearon]

We disabled them in #1742 because of #1638 (webpack/webpack#4423).

[Vanuan]

It looks fine in 2.3.2. Though there are still some non-critical issues with adding a breakpoint in places with 1-to-many lines tranformation.

[gaearon]

Yea, I think we actually merged a PR that re-enables them in master.

[Timer]

I believe one-to-many relations have always been a problem, but it's not blocking.

[Timer]

They were re-enabled in #1894. 😄