#133 changed the browser compatibility matrix of our compat builds. It was a good decision for Fluent and for the web. Unfortunately, while most browser support modern JavaScript, it turns out a significant portion of build tools like Webpack and more specifically, UglifyJS, do not 🤦‍♂️ .

I'm afraid we might need to re-implement strict ES5 compatibility in one way or another. I don't think we can expect existing projects to go through the whole ordeal of updating their build pipeline just to be able to use a newer version of fluent and friends.

Right now we produce two builds for each package:

• index - no transpilation at all, and
• compat - transpiled according to the modern compatibility matrix.

The moder compatiblity matrix expects the engine to support async functions, classes and generators, among others.

I propose we switch to the following strategy:

• index - transpiled according to the modern compatibility matrix,
• compat - transpiled to strictly ES5-compatible code,
• vanilla - not transpiled at all. This could also be called modern or ecma.

The vanilla builds will be allowed to use cutting-edge features like async iteration, implemented only in Firefox 57 and Chrome 63.