[gen_l10n] isCustomDateFormat accepts an actual boolean (#153439)

Fixes #153420

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

@@ -283,6 +283,9 @@ class Placeholder {
     if (value == null) {
       return null;
     }
+    if (value is bool) {
+      return value;
+    }
     if (value != 'true' && value != 'false') {
       throw L10nException(
         'The "$attributeName" value of the "$name" placeholder in message $resourceId '

packages/flutter_tools/test/general.shard/generate_localizations_test.dart

@@ -1658,6 +1658,28 @@ import 'output-localization-file_en.dart' deferred as output-localization-file_e
         expect(content, contains(r"DateFormat('asdf o\'clock', localeName)"));
       });
 
+      testWithoutContext('handle arbitrary formatted date with actual boolean', () {
+        setupLocalizations(<String, String>{
+          'en': '''
+{
+  "@@locale": "en",
+  "springBegins": "Spring begins on {springStartDate}",
+  "@springBegins": {
+    "description": "The first day of spring",
+    "placeholders": {
+      "springStartDate": {
+        "type": "DateTime",
+        "format": "asdf o'clock",
+        "isCustomDateFormat": true
+      }
+    }
+  }
+}'''
+        });
+        final String content = getGeneratedFileContent(locale: 'en');
+        expect(content, contains(r"DateFormat('asdf o\'clock', localeName)"));
+      });
+
       testWithoutContext('throws an exception when no format attribute is passed in', () {
         expect(
           () {