Try to deflake service isolate startup

[dnfield]

Speculative fix for flutter/flutter#106753

Re-orders the call to making the isolate runnable to the end, after we set the private fields.

Tested this locally and it seems to work fine, but I had not reproduced the flake locally.

[flutter-dashboard]

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[bkonyi]

Looks reasonable to me, may as well give it a shot :)

[Hixie]

test-exempt: fixing a test

[a-siva]

Did this change actually deflake the service isolate startup issue ?
The call to Dart_IsolateMakeRunnable only sets the runnable state of the isolate, the isolate itself is scheduled for execution on the thread pool after this callback returns.

[dnfield]

No, this did not seem to have any impact on the flake.