Add support for marking Isolates as daemons (at least in Dart VM)

[DrMarcII]

In tests: Isolates are often used to launch processes such as simple servers for use by tests. These then block the dart vm from exiting unless exit is explicitly called (which can cause other problems, such as preventing coverage information from being collected).

[DartBot]

This comment was originally written by @zoechi

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There was a recent addition which allows to start background processes in dart:io (and also kill by process id). Is this what you are looking for?

[DrMarcII]

That starts a full separate process, which is more than I want.

Currently (if exit is not called), the dart vm will exit only when all isolates have stopped executing (I think). I want to be able to tell it to ignore specific isolates when determining if it should exit (allow it to exit normally without exit being called even though there are still executing isolates).

An example of a use case for this is:
 https://github.com/google/dart-sync-socket/blob/master/test/test_http_client.dart

I launch a simple http server in a separate isolate so that I can test my http client. That http server will never exit (and shouldn't need to). This test currently works only because the unittest vm configuration calls exit. However, that causes problems with collecting coverage information, so is likely to change soon. When that changes, this test will never exit. Being able to mark that isolate as a daemon (similar to how Java can mark threads as daemon threads) would keep that test working as expecting.

[sethladd]

Added Area-Library, Library-Isolate, Triaged labels.

[lrhn]

Actually, the VM is currently exiting as soon as the initial isolate exits.
That may change, though, since it's not always what you want.

If that happens, it would make sense to have a way to determine whether an isolate should be considered for keeping the VM alive. If the default is that the initial isolate is not "demonized" and all spawned isolates default to being demons, then the behavior will stay the same - but you can then choose when spawning whether to make the new isolate keep the VM alive while running.

It's actually a good solution to solving the existing behavior without breaking anything.

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cc @iposva-google.