[pkg/vm_service] Handle lack of optimized out variables in interpreter.

Test the evaluation of the variable as normal when interpreted, and
expect the appropriate RPCError when not interpreted.

TEST=pkg/vm_service/test/evaluate_optimized_out_variable_test

Cq-Include-Trybots: luci.dart.try:vm-linux-release-x64-try,vm-dyn-linux-debug-x64-try,vm-dyn-mac-debug-arm64-try
Change-Id: I964b75fcfbd789aa7ee5b02c325d028a1a9394f4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/464441
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>

pkg/vm_service/test/evaluate_optimized_out_variable_test.dart

@@ -58,12 +58,14 @@ final tests = <IsolateTest>[
   (VmService service, IsolateRef isolateRef) async {
     final isolateId = isolateRef.id!;
     try {
-      await service.evaluateInFrame(
+      final dynamic result = await service.evaluateInFrame(
         isolateId,
         1,
         'data.length',
       );
-      fail('Expected evaluateInFrame to throw an RPCError');
+      // Check in case the variable isn't optimized out, e.g., if the code is
+      // interpreted via bytecode instead of compiled to native code.
+      expect(result.valueAsString, '3');
     } on RPCError catch (e) {
       expect(e.code, RPCErrorKind.kExpressionCompilationError.code);
       expect(e.message, 'Expression compilation error');