[vm] Fix gcc build

TEST=ci

Change-Id: Ifb4f451953e3c4d6c42cafb20e992f970853a8a8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/320881
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>
diff --git a/runtime/vm/compiler/frontend/kernel_translation_helper.h b/runtime/vm/compiler/frontend/kernel_translation_helper.h
index 21d3193..1b782da 100644
--- a/runtime/vm/compiler/frontend/kernel_translation_helper.h
+++ b/runtime/vm/compiler/frontend/kernel_translation_helper.h
@@ -1060,6 +1060,8 @@
       case Kind::ArgType:
         return "vm.inferred-arg-type.metadata";
     }
+    UNREACHABLE();
+    return nullptr;
   }
 
   explicit InferredTypeMetadataHelper(KernelReaderHelper* helper,