[native_assets_cli] Protocol exit codes / unhandled exceptions

[dcharkes]

I'm leaning towards specifying hooks will always be run as a separate process, and non-zero exit code means error.

We need to specify whether build.dart is always run as a separate process.
And we need to specify the failure modes for a build. (e.g. 0-zero exit code for an unhandled exception.)
E.g. what happens if a build succeeds at first, and then fails later. Who is responsible for deleting the outdated build_output.yaml? (The Dart / Flutter SDK.)

We could consider using Isolate.spawnUri to invoke build.dart scripts rather than Process.run.

• This would mean build.dart scripts should not call exit(int).
  It would probably also mean we need to think about what to do with unhandled exceptions.
• Doing this would isolate the builds less.
• Doing this might speed up the builds slightly (we'd still need to JIT from source every time, so we'd only be saving the Dart VM C++ process startup)

I believe it's probably best to use process isolation for the builds and not run them as Isolate.spawnUri

[dcharkes]

Also, when addressing this, we should see that the user experience is good when one package build fails when there are multiple packages with native assets.

[dcharkes]

Also, when addressing this, we should see that the user experience is good when one package build fails when there are multiple packages with native assets.

Currently it's broken:

• [native_assets_builder] If running hook/build.dart of a package fails, then we should not continue building dependent packages #1630

[mkustermann]

Currently it's broken:

Better to verify, but I may have fixed that as well in 5164777