Allow add_[format]() in flutter gen-l10n

packages/flutter_tools/lib/src/localizations/gen_l10n.dart

@@ -165,9 +165,20 @@ String generateDateFormattingLogic(Message message) {
         );
       }
       if (placeholder.hasValidDateFormat) {
+        final List<String> formatParts = placeholderFormat.split(dateFormatPartsDelimiter);
+        final String mainFormat = formatParts.first;
+        final List<String> addFormats = formatParts.skip(1).toList();
+
+        // 'format' can contain a number of date time formats separated by dateFormatPartsDelimeter.
+        // The first format is the main format, and the rest are additional formats.
+        final String addedFormatsString = addFormats.map((String addFormat) {
+          return dateFormatAddFormatTemplate.replaceAll('@(format)', addFormat);
+        }).join();
+
         return dateFormatTemplate
           .replaceAll('@(placeholder)', placeholder.name)
-          .replaceAll('@(format)', placeholderFormat);
+          .replaceAll('@(format)', mainFormat)
+          .replaceAll('@(addFormats)', addedFormatsString);
       }
       return dateFormatCustomTemplate
         .replaceAll('@(placeholder)', placeholder.name)

packages/flutter_tools/lib/src/localizations/gen_l10n_templates.dart

@@ -123,10 +123,12 @@ const String numberFormatNamedTemplate = '''
 ''';
 
 const String dateFormatTemplate = '''
-    final intl.DateFormat @(placeholder)DateFormat = intl.DateFormat.@(format)(localeName);
+    final intl.DateFormat @(placeholder)DateFormat = intl.DateFormat.@(format)(localeName)@(addFormats);
     final String @(placeholder)String = @(placeholder)DateFormat.format(@(placeholder));
 ''';
 
+const String dateFormatAddFormatTemplate = '''.add_@(format)()''';
+
 const String dateFormatCustomTemplate = '''
     final intl.DateFormat @(placeholder)DateFormat = intl.DateFormat(@(format), localeName);
     final String @(placeholder)String = @(placeholder)DateFormat.format(@(placeholder));

packages/flutter_tools/lib/src/localizations/gen_l10n_types.dart

@@ -72,6 +72,8 @@ const Set<String> validDateFormats = <String>{
   's',
 };
 
+const String dateFormatPartsDelimiter = '+';
+
 // The set of number formats that can be automatically localized.
 //
 // The localizations generation tool makes use of the intl library's
@@ -252,7 +254,7 @@ class Placeholder {
   bool get requiresNumFormatting => <String>['int', 'num', 'double'].contains(type) && format != null;
   bool get hasValidNumberFormat => _validNumberFormats.contains(format);
   bool get hasNumberFormatWithParameters => _numberFormatsWithNamedParameters.contains(format);
-  bool get hasValidDateFormat => validDateFormats.contains(format);
+  bool get hasValidDateFormat => format?.split(dateFormatPartsDelimiter).every(validDateFormats.contains) ?? false;
 
   static String? _stringAttribute(
     String resourceId,