Export __esModule when bundling to CJS or UMD in named mode.

[cameron-martin]

When the format option is set to 'umd' or 'cjs' and the exports option is set to 'named', we should automatically export the __esModule property (with value true).

This means that a library compiled using rollup can be imported using an app which uses ES6 syntax for imports, then compiles to CommonJS using babel.

Can anyone see any downsides to doing this? If not, I'd be happy to submit a pull request.

[eventualbuddha]

We should do that. I've been meaning to for a long time.

[Rich-Harris]

This is enabled in 0.26.7 – thanks

[ericf]

I've started receiving some issues people ran into with because of this change. They were incorrectly importing the default export of React Intl which does not have a default export, then accessing a named export off of it.

• 2.1.3 throws exception (addLocaleData)..  formatjs/formatjs#499
• Breaking change introduced in v2.1.3! formatjs/formatjs#503

Obviously the user's import statements are incorrect, but I just wanted to point this out since @cameron-martin asked about potential downsides.