Coverage is slow on large projects

[dnfield]

Corresponds to internal bug b/179848548

See also flutter/flutter#81803

Getting coverage from the VM service involves multiple RPC calls that transfer megabytes worth of data per test. This becomes untenable for larger projects with lots of source files and lots of tests.

It seems like we could get a lot of mileage out of a single VM call (or flag) that would generate a more streamlined coverage file (perhaps even just an lcov file).

This bug blocks Flutter from being able to reasonably run coverage on CI without timing out or using a very very powerful machine.

/cc @twobitunicorn fyi
/cc @liamappelbe @a-siva

[IchordeDionysos]

This might also block adding function and branch coverage, as I understand this also would require adding new RPC calls (at least for branch coverage).

See dart-lang/coverage#141 for more information.

[a-siva]

Most of the discussion on improving coverage speed is happening in flutter/flutter#86722

[a-siva]

@liamappelbe is more work expected on this issue ?

[liamappelbe]

No. flutter/flutter#90142 halves the time it takes to gather coverage. And isolate groups have also helped because we only need to gather coverage once for the whole group. There's no other low hanging fruit I'm aware of, so we should close this and reopen if users are still having problems.

[creativecreatorormaybenot]

@liamappelbe did you consider cherry-picking this into Flutter stable?