Problem

Flutter Web currently generates several JS files to integrate with the browser, like flutter.js and flutter_service_worker.js.

While doing changes to those files, it's some times hard to remember what bits of ECMAScript are supported by which versions of each browser, and bugs like this happen:

• [tool][web] Fix flutter.js in Safari 13 #104761
• use old Edge-compatible JavaScript in index.html #83732

(And possibly others)

Proposal

Integrate in our build process something like esbuild that would let us:

• Write the JavaScript in whatever version we want, with the latest features
• Ensure that said javascript will run in our supported browsers (with --target=es6, for example), regardless of what features we use
• (optional) Minify the code.

PS: the esbuild pass could also happen as post-processing after a flutter build web, this doesn't need to be necessarily integrated with the flutter tool, so adding the "this may be a good package" label also.