Fix the extract method refactoring to correctly handle nullable return types

Change-Id: I2a885887a8d871f0c61afd6648843b914c59022d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/204626
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

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

@@ -17,6 +17,7 @@ import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/precedence.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/dart/element/nullability_suffix.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/error/listener.dart';
 import 'package:analyzer/src/dart/ast/extensions.dart';
@@ -865,6 +866,7 @@ class CorrectionUtils {
       return _getTypeCodeElementArguments(
         librariesToImport: librariesToImport,
         element: aliasElement,
+        isNullable: type.nullabilitySuffix == NullabilitySuffix.question,
         typeArguments: aliasArguments,
       );
     }
@@ -895,6 +897,7 @@ class CorrectionUtils {
       return _getTypeCodeElementArguments(
         librariesToImport: librariesToImport,
         element: type.element,
+        isNullable: type.nullabilitySuffix == NullabilitySuffix.question,
         typeArguments: type.typeArguments,
       );
     }
@@ -1161,6 +1164,7 @@ class CorrectionUtils {
   String? _getTypeCodeElementArguments({
     required Set<Source> librariesToImport,
     required Element element,
+    required bool isNullable,
     required List<DartType> typeArguments,
   }) {
     var sb = StringBuffer();
@@ -1188,6 +1192,9 @@ class CorrectionUtils {
     // append simple name
     var name = element.displayName;
     sb.write(name);
+    if (isNullable) {
+      sb.write('?');
+    }
 
     // append type arguments
     if (typeArguments.isNotEmpty) {

pkg/analysis_server/test/edit/refactoring_test.dart

@@ -763,6 +763,43 @@ void res(int a, int b) {
 ''');
   }
 
+  Future<void> test_statements_nullableReturnType() {
+    addTestFile('''
+void foo(int b) {
+// start
+  int? x;
+  if (b < 2) {
+    x = 42;
+  }
+  if (b >= 2) {
+    x = 43;
+  }
+// end
+  print(x!);
+}
+''');
+    _setOffsetLengthForStartEnd();
+    return assertSuccessfulRefactoring(_computeChange, '''
+void foo(int b) {
+// start
+  int? x = res(b);
+// end
+  print(x!);
+}
+
+int? res(int b) {
+  int? x;
+  if (b < 2) {
+    x = 42;
+  }
+  if (b >= 2) {
+    x = 43;
+  }
+  return x;
+}
+''');
+  }
+
   Future<Response> _computeChange() async {
     await _prepareOptions();
     // send request with the options

pkg/analysis_server/test/services/refactoring/extract_method_test.dart

@@ -1209,7 +1209,7 @@ f(bool p) {
     _createRefactoringForStartEndComments();
     // do check
     await refactoring.checkInitialConditions();
-    expect(refactoring.returnType, 'int');
+    expect(refactoring.returnType, 'int?');
   }
 
   Future<void> test_returnType_statements_void() async {