Fix issue #156954: Wrong dateHelpText in MaterialLocalizationIt (#161889)

Fix #156954
- Changed current value `dataHelpText` used in `MaterialLocalizationIt`
from "mm/gg/aaaa" to "gg/mm/aaaa"
- Wrote test

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packages/flutter_localizations/lib/src/l10n/generated_material_localizations.dart

@@ -21688,7 +21688,7 @@ class MaterialLocalizationIt extends GlobalMaterialLocalizations {
   String get cutButtonLabel => 'Taglia';
 
   @override
-  String get dateHelpText => 'mm/gg/aaaa';
+  String get dateHelpText => 'gg/mm/aaaa';
 
   @override
   String get dateInputLabel => 'Inserisci data';

packages/flutter_localizations/lib/src/l10n/material_it.arb

@@ -56,7 +56,7 @@
   "refreshIndicatorSemanticLabel": "Aggiorna",
   "moreButtonTooltip": "Altro",
   "dateSeparator": "/",
-  "dateHelpText": "mm/gg/aaaa",
+  "dateHelpText": "gg/mm/aaaa",
   "selectYearSemanticsLabel": "Seleziona anno",
   "unspecifiedDate": "Data",
   "unspecifiedDateRange": "Intervallo di date",

packages/flutter_localizations/test/material/translations_test.dart

@@ -596,4 +596,15 @@ void main() {
     expect(localizations.copyButtonLabel, '복사');
     expect(localizations.pasteButtonLabel, '붙여넣기');
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/156954
+  testWidgets('Italian translation for dateHelpText', (WidgetTester tester) async {
+    const Locale locale = Locale('it');
+    expect(GlobalCupertinoLocalizations.delegate.isSupported(locale), isTrue);
+    final MaterialLocalizations localizations = await GlobalMaterialLocalizations.delegate.load(
+      locale,
+    );
+    expect(localizations, isA<MaterialLocalizationIt>());
+    expect(localizations.dateHelpText, 'gg/mm/aaaa');
+  });
 }