Add --warn-on-legacy-js-interop flag to JS compilers (dart2js & DDC)

[kevmoo]

Talking to @srujzs about this.

We should let folks OPT-IN to getting warnings here. We can use this on critical packages, etc in the short term to start pushing folks towards wasm-compatible JS-interop model.

CC @sigmundch ... and @joshualitt

[rakudrama]

Could this be a lint? That has everything needed to manage opt-in.

[kevmoo]

@rakudrama – great idea! I'd take it.

The only issue with a lint: it's not reported for ALL CODE.

I want to be able to compile https://gallery.flutter.dev/ – or some other big app – and see all of the spots that would need to get fixed to run w/ wasm (at least in theory)

[kevmoo]

See also #58957

[kevmoo]

Going to go with the lint idea. It's cleaner.