[webdev] Generate auto-switching combined build for dart2js and dart2wasm output

[parlough]

I'm not exactly sure at what layer this should be in, but webdev seems like an most appropriate place. Happy to hear what other's think! Perhaps it does make sense directly as a dart subcommand, not sure.

Building on dart-lang/sdk@0df58fc and eventually dart-lang/build#3621:

I think webdev should have a release mode that generates output with both dart2js (dart compile js) and dart2wasm (dart compile wasm). Then it should generate a JS script which chooses between them based on browser capabilities.

Developers can technically do this themselves, but I imagine it will be common enough that it's worth having some built-in functionality to do so.

Perhaps for a while, this can be a separate mode from the existing --release, but eventually I'd expect this to be the default.

\cc @kevmoo @jakemac53 For general thoughts, including about where this functionality could live.

[jakemac53]

This probably has to be a part of the build package explicitly, because usually we don't support compiling the same app with multiple compilers (as a part of a single build). We could though, and I had just yesterday started some informal conversations regarding how we should do the switching too, as a part of dart-lang/build#3621 :).

[mit-mit]

Functionally I think we'd want something like this:

• webdev serve: Do DDC compilation + serve up result
• webdev build: Do dart2js & dart2wasm compilation; plus bundle up in some out folder with bootstrapping logic that selects between JS and Wasm in the browser.

wdyt @jakemac53 @kevmoo ?

[jakemac53]

That seems pretty reasonable, and I was looking into doing this before, but I need good direction from the dart2js/wasm teams on how to implement the JS/Wasm switching.

[kevmoo]

@jakemac53 – @eyebrowsoffire has figured this out for Flutter. We should look to share logic as much as possible!

[bkonyi]

@kevmoo @mit-mit is this something that we should prioritize?

[kevmoo]

@bkonyi – I'd LOVE to have this.

And we have the first step here: dart-lang/build#3727

CC @jakemac53 who has been helping on the review side

[mit-mit]

Yeah, +1, this would be great to enable the ecosystem

[simolus3]

I have also opened dart-lang/build#3737 as a follow-up for build_web_compilers to optionally compile with both compilers and then generate an entrypoint script loading the correct entrypoint based on browser features.
I don't think there's anything that webdev would have to do in addition to that.