Add Message.isPlural, Message.getCountPlaceholder() to gen_l10n_types (#50132)

dev/tools/localization/gen_l10n.dart

@@ -286,9 +286,17 @@ String generatePluralMethod(Message message) {
     'other': 'other'
   };
 
-  final String countPlaceholder = message.value.split(',')[0].substring(1);
+  final Placeholder countPlaceholder = message.getCountPlaceholder();
+  if (countPlaceholder == null) {
+    throw L10nException(
+      'Unable to find the count placeholder for the plural message: ${message.resourceId}.\n'
+      'Check to see if the plural message is in the proper ICU syntax format '
+      'and ensure that placeholders are properly specified.'
+    );
+  }
+
   final List<String> intlMethodArgs = <String>[
-    countPlaceholder,
+    countPlaceholder.name,
     'locale: _localeName',
     ...generateIntlMethodArgs(message),
   ];
@@ -311,7 +319,7 @@ String generatePluralMethod(Message message) {
 
   List<String> generatePluralMethodParameters([String type]) {
     return message.placeholders.map((Placeholder placeholder) {
-      final String placeholderType = placeholder.name == countPlaceholder ? 'int' : (type ?? placeholder.type);
+      final String placeholderType = placeholder == countPlaceholder ? 'int' : (type ?? placeholder.type);
       return '$placeholderType ${placeholder.name}';
     }).toList();
   }
@@ -346,7 +354,6 @@ class LocalizationsGenerator {
 
   static RegExp arbFilenameLocaleRE = RegExp(r'^[^_]*_(\w+)\.arb$');
   static RegExp arbFilenameRE = RegExp(r'(\w+)\.arb$');
-  static RegExp pluralValueRE = RegExp(r'^\s*\{[\w\s,]*,\s*plural\s*,');
 
   final file.FileSystem _fs;
 
@@ -673,7 +680,7 @@ class LocalizationsGenerator {
       }
 
       final Message message = Message(bundle, key);
-      if (pluralValueRE.hasMatch(message.value))
+      if (message.isPlural)
         classMethods.add(generatePluralMethod(message));
       else
         classMethods.add(genSimpleMethod(message));

dev/tools/localization/gen_l10n_types.dart

@@ -81,15 +81,32 @@ class Message {
       assert(resourceId != null && resourceId.isNotEmpty),
       value = _value(bundle, resourceId),
       description = _description(bundle, resourceId),
-      placeholders = _placeholders(bundle, resourceId);
+      placeholders = _placeholders(bundle, resourceId),
+      _pluralMatch = _pluralRE.firstMatch(_value(bundle, resourceId));
+
+  static final RegExp _pluralRE = RegExp(r'\s*\{([\w\s,]*),\s*plural\s*,');
 
   final String resourceId;
   final String value;
   final String description;
   final List<Placeholder> placeholders;
+  final RegExpMatch _pluralMatch;
+
+  bool get isPlural => _pluralMatch != null && _pluralMatch.groupCount == 1;
 
   bool get placeholdersRequireFormatting => placeholders.any((Placeholder p) => p.requiresFormatting);
 
+  Placeholder getCountPlaceholder() {
+    assert(isPlural);
+    final String countPlaceholderName = _pluralMatch[1];
+    return placeholders.firstWhere(
+      (Placeholder p) => p.name == countPlaceholderName,
+      orElse: () {
+        throw L10nException('Cannot find the $countPlaceholderName placeholder in plural message "$resourceId".');
+      }
+    );
+  }
+
   static String _value(Map<String, dynamic> bundle, String resourceId) {
     final dynamic value = bundle[resourceId];
     if (value == null)