[dart_runner] Properly handle non-error Isolate exit

Differentiate an Isolate going down from calling exit()
from package:fuchsia from an Isolate going down due
to an error

Needs: https://fuchsia-review.googlesource.com/c/build/+/155166

FL-51 #done

Change-Id: If2e37f6eac1ab19472ab410a72e3961d9aeaf517
1 file changed