[dart2js] Implement support for dynamic checks on parameters that are covariant-by-declaration due to inheritance

[eernstg]

This is the dart2js specific issue for #47072, which has the details.

[devoncarew]

@rakudrama (or @sigmundch) - can you provide a very rough estimate of how long this would take to do? We're trying to get breadbox estimates of the ~5 impl. tasks for this language change.

(updated to reference the correct github account)

[sigmundch]

@rakudrama (our apologies for the spam @sra)

[rakudrama]

It would be useful to have a sketch of how the CFE will present the additional checks.

[devoncarew]

cc @johnniwinther

[johnniwinther]

I'm working on a CL that'll add explicit AsExpressions to the already generated forwarding stub. Some of the stubs are skipped by dart2js so we might need to fix that.

[devoncarew]

@sigmundch - can you assign the right owner for this? Thanks!

[sigmundch]

Thanks @devoncarew, will follow up shortly after our team sync.

@johnniwinther - are the CFE representation changes in place?

[sigmundch]

quick update - @johnniwinther will be looking first at whether the lowering in the CFE needs any additional work, and will reassign it to us once it's ready for us to take a look.