Smart scheduling dependent targets

[keyonghan]

Flutter infra team has been spending quarters effort to scale running tasks. One big milestone is separating tasks to subbuilds to improve efficiency and enhance reusability.

• Engine targets are now being migrated to engine_v2, which separates different logics into their own self-contained steps: build, test, generator, archive. flutter.dev/go/engine-build-definition-language
• Framework devicelab targets are now being migrated to build+test models go/devicelab-build-test

The current scheduling workflow however is bearing with several issues:

• it needs an orchestrator build to schedule all subbuilds
• any single subbuild fails will cause the whole orchestrator build failure
• the latter subbuild can only be triggered after all artifacts are finished building, though may need only one of them

This bug tracks a smart graph scheduling logic for these dependent targets to save resources and validation time.

[CaseyHillers]

From the Cocoon side, I drafted flutter/cocoon#2391 and a one pager.

[flutter-triage-bot]

This issue is assigned to @keyonghan but has had no recent status updates. Please consider unassigning this issue if it is not going to be addressed in the near future. This allows people to have a clearer picture of what work is actually planned. Thanks!

[flutter-triage-bot]

This issue was assigned to @keyonghan but has had no status updates in a long time. To remove any ambiguity about whether the issue is being worked on, the assignee was removed.