Update to a newer version of Fluent

[stasm]

The Test Pilot website uses two Fluent dependencies: the core fluent package as well as fluent-react, the bindings for React. In #3631 I mentioned that I'm working on a fix to fluent-react.

Independently of fluent-react, I'd like Test Pilot to update to a newer version of the core fluent package. You're currently o n 0.4.3 and the latest is 0.7.0.

I tried updating it in my local setup, and unfortunately I ran into a build error. One of the changes 0.7.0 shipped with is an updated compatibility matrix for the fluent/compat builds. And while this new matrix is aligned with your browser support documented in the FAQ, it turns out it's not compatible with old versions of Webpack. More specifically, UglifyJS breaks on non-ES5 syntax 🤦‍♂️ (e.g. class or async).

I'm filing this issue to let you know of my intent to update fluent somehow and to track my progress on dealing with the build error. And also to ask if you have any plans to upgrade to a newer version of Webpack and Uglify? :)

[clouserw]

Marking this small assuming an ES5 build will be provided