Generate valid functions with optional non-nullable arguments

If a method of a mocked class returns a function with optional
yet non-nullable argument, Mockito currently produces something
like
```dart
(int x, {int y}) => /* doesn't matter */
```
which is not a valid Dart, since `y` must either be `required` or
have a default value. `required` won't fit into the desired type
and coming up with a default value is not easy, since it has to
be a constant, but `dummyValue` doesn't always produce one.

Fortunately, we can just widen the type here and make it nullable.

PiperOrigin-RevId: 582669927

lib/src/builder.dart

@@ -1580,12 +1580,13 @@ class _MockClassInfo {
           } else if (parameter.isOptionalPositional) {
             final matchingParameter = _matchingParameter(parameter,
                 superParameterType: parameter.type,
-                defaultName: '__p$position');
+                defaultName: '__p$position',
+                forceNullable: true);
             b.optionalParameters.add(matchingParameter);
             position++;
           } else if (parameter.isOptionalNamed) {
             final matchingParameter = _matchingParameter(parameter,
-                superParameterType: parameter.type);
+                superParameterType: parameter.type, forceNullable: true);
             b.optionalParameters.add(matchingParameter);
           } else if (parameter.isRequiredNamed) {
             final matchingParameter = _matchingParameter(parameter,

test/builder/auto_mocks_test.dart

@@ -2414,7 +2414,7 @@ void main() {
       _containsAllOf('''
         returnValue: (
           int __p0, [
-          String __p1,
+          String? __p1,
         ]) {},'''),
     );
   });
@@ -2431,7 +2431,7 @@ void main() {
       _containsAllOf('''
         returnValue: (
           _i2.Foo __p0, {
-          bool b,
+          bool? b,
         }) {},'''),
     );
   });

test/end2end/foo.dart

@@ -22,6 +22,9 @@ class Foo<T> {
   Future<void>? returnsNullableFutureVoid() => Future.value();
   Future<T> returnsFuture(T x) => Future.value(x);
   Bar returnsBar(int arg) => Bar();
+  String Function(int x, [String s]) returnsFunction() => (x, [s = '']) => '';
+  String Function(int x, {String s}) returnsFunctionNamed() =>
+      (x, {s = ''}) => '';
 }
 
 class Bar {

test/end2end/generated_mocks_test.dart

@@ -163,6 +163,22 @@ void main() {
       when(foo.returnsFuture(any)).thenAnswer((_) async => 1);
       expect(await foo.returnsFuture(0), 1);
     });
+
+    test(
+        'a method returning a function with optional non-nullable argument '
+        'can be stubbed', () {
+      when(foo.returnsFunction()).thenReturn((x, [s = 'x']) => s + s);
+      expect(foo.returnsFunction()(1), equals('xx'));
+      expect(foo.returnsFunction()(1, 'y'), equals('yy'));
+    });
+
+    test(
+        'a method returning a function with named optional non-nullable '
+        'argument can be stubbed', () {
+      when(foo.returnsFunctionNamed()).thenReturn((x, {s = 'x'}) => s + s);
+      expect(foo.returnsFunctionNamed()(1), equals('xx'));
+      expect(foo.returnsFunctionNamed()(1, s: 'y'), equals('yy'));
+    });
   });
 
   group('for a generated mock using unsupportedMembers', () {