Add retry flag to flutter_test (#125851)

Closes https://github.com/flutter/flutter/issues/125920

I will add tests, polish code, etc, if this change looks generally OK!

packages/flutter_test/lib/src/test_compat.dart

@@ -185,8 +185,8 @@ void test(
 /// should explain why the group is skipped; this reason will be printed instead
 /// of running the group's tests.
 @isTestGroup
-void group(Object description, void Function() body, { dynamic skip }) {
+void group(Object description, void Function() body, { dynamic skip, int? retry }) {
-  _declarer.group(description.toString(), body, skip: skip);
+  _declarer.group(description.toString(), body, skip: skip, retry: retry);
 }
 
 /// Registers a function to be run before tests.

packages/flutter_test/lib/src/widget_tester.dart

@@ -130,6 +130,7 @@ void testWidgets(
   bool semanticsEnabled = true,
   TestVariant<Object?> variant = const DefaultTestVariant(),
   dynamic tags,
+  int? retry,
 }) {
   assert(variant.values.isNotEmpty, 'There must be at least one value to test in the testing variant.');
   final TestWidgetsFlutterBinding binding = TestWidgetsFlutterBinding.ensureInitialized();
@@ -174,6 +175,7 @@ void testWidgets(
       skip: skip,
       timeout: timeout ?? binding.defaultTestTimeout,
       tags: tags,
+      retry: retry,
     );
   }
 }

packages/flutter_test/test/widget_tester_test.dart

@@ -51,6 +51,24 @@ void main() {
     });
   });
 
+  group('group retry flag allows test to run multiple times', () {
+    bool retried = false;
+    group('the group with retry flag', () {
+      testWidgets('the test inside it', (WidgetTester tester) async {
+        addTearDown(() => retried = true);
+        expect(retried, isTrue);
+      });
+    }, retry: 1);
+  });
+
+  group('testWidget retry flag allows test to run multiple times', () {
+    bool retried = false;
+    testWidgets('the test with retry flag', (WidgetTester tester) async {
+      addTearDown(() => retried = true);
+      expect(retried, isTrue);
+    }, retry: 1);
+  });
+
   group('respects the group skip flag', () {
     testWidgets('should be skipped', (WidgetTester tester) async {
       expect(false, true);