[dartdevc] Fix analysis error in runtime library

Change-Id: I4aa4a580bc8fd1c7db96578a9e715e191af7fbda
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/131500
Reviewed-by: Vijay Menon <[email protected]>
Reviewed-by: Nate Bosch <[email protected]>
Commit-Queue: Vijay Menon <[email protected]>

Author: nshahan

sdk_nnbd/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart

@@ -1494,7 +1494,7 @@ class _TypeInferrer {
       }
     }
     for (var name in supertypeRequiredNamed.keys) {
-      var subtypeParamType = subtypeRequiredNamed[name] ?? subtypeNamed[name];
+      var subtypeParamType = subtypeRequiredNamed[name] ?? subtypeNamed[name]!;
       if (!_isSubtypeMatch(supertypeRequiredNamed[name]!, subtypeParamType)) {
         return false;
       }