[cfe] Handle promotion of Null to Never

Change-Id: If784cc315e0ea179e98df45a06d4a99179e4c197
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/127140
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

Author: johnniwinther

pkg/_fe_analyzer_shared/test/flow_analysis/type_promotion/data/null_check.dart

@@ -17,3 +17,11 @@ promotesNullableType(int? x) {
     x;
   }
 }
+
+promotesNullType(Null x) {
+  if (x != null) {
+    /*Never*/ x;
+  } else {
+    x;
+  }
+}

pkg/front_end/lib/src/fasta/type_inference/type_inference_engine.dart

@@ -14,6 +14,7 @@ import 'package:kernel/ast.dart'
         InterfaceType,
         Member,
         NamedType,
+        NeverType,
         Nullability,
         TreeNode,
         TypeParameter,
@@ -267,6 +268,8 @@ class TypeOperationsCfe
             type.parameter, type.typeParameterTypeNullability, bound);
       }
       return type;
+    } else if (type == typeEnvironment.nullType) {
+      return const NeverType(Nullability.nonNullable);
     }
     return type.withNullability(Nullability.nonNullable);
   }