Use es2015-loose Babel preset in production build

Almost every React package that I’m aware of uses the loose version of the preset to transpile, given that es2015-loose produces code that’s easier to understand and more performant.

As such, it would probably make the most sense to use es2015-loose at least for the production Babel config. There are a few edge cases where it allows illegal code to work, so it may not be appropriate for the development config, but I think any “serious” production Babel config should be expected to use es2015-loose rather than es2015.

[gaearon]

If we enable loose, we enable it everywhere. Having differences between dev and prod is bad.

[taion]

Worth noting that as-is, es2015 produces code that can’t run in ES5 environments without polyfills – https://phabricator.babeljs.io/T7518

es2015-loose doesn’t have this problem.