Ensure correct child isolate initialization in enable-isolate-groups dart vm configuration.

[aam]

Dart_SetRootLibrary and LoadKernel needs to be done only for a root isolate in the isolate group. When in --enable-isolate-groups dart vm mode child isolates will reuse kernel and object store set up by root isolate.

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[aam]

cc @mkustermann regarding changes to isolate loading for ltw isolates: avoid loading kernel for children isolates

[chinmaygarde]

Just curious about progress on this patch. Is there some blocker?

[mkustermann]

@chinmaygarde

This change will be required iff --enable-isolate-groups is also enabled in JIT mode. So turning on the flag will require these changes.

We could land the PR now (despite being the flag off atm) by guarding the changes via Dart_IsVMFlagSet("--enable-isolate-groups"). Then once flipping the flag, it will do the right thing. Is that something you prefer?

[mkustermann]

@aam Could you update this PR to guard the changes under the Dart_IsVMFlagSet("--enable-isolate-groups") condition? That would make it easy to enable the flag (and if something goes wrong, disable it again without reverting bigger changes)

[aam]

@aam Could you update this PR to guard the changes under the Dart_IsVMFlagSet("--enable-isolate-groups") condition? That would make it easy to enable the flag (and if something goes wrong, disable it again without reverting bigger changes)

Done, but I left the code in dart_vm.cc that enables the flag too.

[mkustermann]

Done, but I left the code in dart_vm.cc that enables the flag too.

@aam Could you limit this change to only avoid the kernel loading (i.e. not enable the flag, not roll DEPS and all the other changes)?

I think we will avoid flipping the flag here in the embedder and instead turn it on by-default in the VM instead, which will transparently make us use the right code path here.

[aam]

@aam Could you limit this change to only avoid the kernel loading (i.e. not enable the flag, not roll DEPS and all the other changes)?

There should be only four files changed in this PR(runtime/dart_isolate.cc, runtime/dart_isolate.h, runtime/dart_vm.cc, runtime/isolate_configuration.cc).
Do you see more(DEPS, anything else)?

[aam]

I think we will avoid flipping the flag here in the embedder

Done.

[mkustermann]

Do you see more(DEPS, anything else)?

I did. Not sure why, but after your latest push the Files changed shows the right thing.