DAS: Use correct nullability in add_return_type

Fixes https://github.com/dart-lang/sdk/issues/54865

The nullability of return types of async and generator functions is not
based on the nullability of the returned or yielded values.

Change-Id: Ieae534dbf64135129b9891b7db047f5617401021
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/501121
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Keerti Parthasarathy <keertip@google.com>

pkg/analysis_server/lib/src/services/correction/dart/add_return_type.dart

@@ -18,8 +18,7 @@ class AddReturnType extends ResolvedCorrectionProducer {
   AddReturnType({required super.context});
 
   @override
-  CorrectionApplicability get applicability =>
-      CorrectionApplicability.automatically;
+  CorrectionApplicability get applicability => .automatically;
 
   @override
   AssistKind get assistKind => DartAssistKind.addReturnType;
@@ -98,18 +97,18 @@ class AddReturnType extends ResolvedCorrectionProducer {
       if (body.isGenerator) {
         return typeProvider.streamElement.instantiate(
           typeArguments: [baseType],
-          nullabilitySuffix: baseType.nullabilitySuffix,
+          nullabilitySuffix: .none,
         );
       } else {
         return typeProvider.futureElement.instantiate(
           typeArguments: [baseType],
-          nullabilitySuffix: baseType.nullabilitySuffix,
+          nullabilitySuffix: .none,
         );
       }
     } else if (body.isGenerator) {
       return typeProvider.iterableElement.instantiate(
         typeArguments: [baseType],
-        nullabilitySuffix: baseType.nullabilitySuffix,
+        nullabilitySuffix: .none,
       );
     }
     return baseType;

pkg/analysis_server/test/src/services/correction/assist/add_return_type_test.dart

@@ -184,6 +184,45 @@ String f() {
 ''');
   }
 
+  Future<void> test_topLevelFunction_block_async() async {
+    await resolveTestCode('''
+^f(String? s) async {
+  return s;
+}
+''');
+    await assertHasAssist('''
+Future<String?> f(String? s) async {
+  return s;
+}
+''');
+  }
+
+  Future<void> test_topLevelFunction_block_asyncStar() async {
+    await resolveTestCode('''
+^f(String? s) async* {
+  yield s;
+}
+''');
+    await assertHasAssist('''
+Stream<String?> f(String? s) async* {
+  yield s;
+}
+''');
+  }
+
+  Future<void> test_topLevelFunction_block_syncStar() async {
+    await resolveTestCode('''
+^f(String? s) sync* {
+  yield s;
+}
+''');
+    await assertHasAssist('''
+Iterable<String?> f(String? s) sync* {
+  yield s;
+}
+''');
+  }
+
   Future<void> test_topLevelFunction_expression() async {
     await resolveTestCode('''
 ^f() => '';