Babel interopRequireWildcard helper reexports

[guybedford]

This adds support for reexport patterns based on var _x = _interopRequireWildcard(require('x')).

We specifically detect the "_interopRequireWildcard" identifier, so like TypeScript helper reexports this will not survive minification, but for npm packages the detection goal of supporting transpiled ES modules is still met despite this potential issue.

Fixes #52

/cc @nicolo-ribaudo

[nicolo-ribaudo]

I feel like detecting interopRequireWildcard specifically is fragile, but:

1. I can't think of a better pattern
2. Most modules on npm aren't minified anyway

[guybedford]

@nicolo-ribaudo how reliable is the _interopRequireWildcard name here (with the _)? I know it's fragile, it would just be nice to get a sense of how fragile. Should we support without the leading _, and possibly with $n?

[nicolo-ribaudo]

Babel will never inject it without the leading _.

$n could happen when you compile a file which has already been compiled (thus it already contains an _interopRequireWildcard function). When people compile re-compile files it's almost always because they are bundling and compiling their dependencies: in this case Babel's exports wouldn't be the final bundle exports anyway. I don't think we need to worry about it.

[guybedford]

Thanks for confirming!