refactor test shards

[devoncarew]

• update the test sharding logic to use the dart test --total-shards and --shard-index cli args

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[sigurdm]

Just for historical context, this didn't work before dart-lang/test#1452 was solved.

[sigurdm]

Thanks!

[devoncarew]

Just for historical context, this didn't work before dart-lang/test#1452 was solved.

Ah, cool. Yeah, I saw the bash sharding config in the workflow file and assumed it pre-dated the dart test sharding support. I sent this PR to see if things would generally work using that built-in support - it makes the workflow file a bit simplier.