[nnbd] Fix small strong mode bug in service isolate
[nnbd] Fix small strong mode bug in service isolate

Fixes:
lib/mirrors/invocation_fuzz_test/emptyarray
lib/mirrors/invocation_fuzz_test/false
lib/mirrors/invocation_fuzz_test/none
Change-Id: If107cbbdb36fb8565c4d3aff563ab52a7a632616
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/144990
Reviewed-by: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>
Commit-Queue: Liam Appelbe <78c2029e58e395825ba7228e218c841bb436a9e9@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk