"pub run" should spawn an isolate rather than starting a subprocess

[nex3]

Currently "pub run" starts a subprocess to run its code. It should instead spawn an isolate to avoid VM startup time and provide accurate information for the stdioType getter.

[nex3]

Added Pub-Run label.

[nex3]

This is blocking dart-lang/test#19.

[DartBot]

This comment was originally written by @seaneagan

---

In the meantime could pub run temporarily set an environment variable in the sub-process for us to check the stdioType e.g. PUB_RUN_STDIOTYPE?

This is also blocking https://github.com/google/stagehand/issues/147

[DartBot]

This comment was originally written by @seaneagan

---

This would fix stdioType, but wouldn't it break Platform.script?

[nex3]

Platform.script is already not what people expect under "pub run". Ideally there would be a way to spawn an isolate with a custom value for Platform.script, but that feature request is out-of-scope for this issue.

[DartBot]

This comment was originally written by @seaneagan

---

I think Platform.script currently references the pub cache location of the script, no? I only need basenameWithoutExtension(Platform.script), so that has been sufficient for my needs, and I'm not sure what a better value would be. But I would no longer have that if the isolate retains the Platform.script of pub itself, rather than of the spawned script.

(BTW my use case is for unscripted to determine the command name for help text output.)

[DartBot]

This comment was originally written by @seaneagan

---

See also issue #22098, which would be an even better solution for my use case.

[nex3]

This is also blocking dart-lang/test#86.

[DartBot]

This comment was originally written by emil.n.pe...@gmail.com

---

Case where this is a real problem:
"stdout.terminalColumns" throws when file is run with "pub run" rather than "dart"

[DartBot]

This issue has been moved to dart-lang/pub#1204.