fix return type comment parsing for top-level generic function

pkg/analyzer/lib/src/generated/parser.dart

Index: pkg/analyzer/lib/src/generated/parser.dart
diff --git a/pkg/analyzer/lib/src/generated/parser.dart b/pkg/analyzer/lib/src/generated/parser.dart
index 42872c149d6a97089c020f10c31c2106b6faef4f..20745254b2dc6f198980ca2ee5d0756d4e9c93ee 100644
--- a/pkg/analyzer/lib/src/generated/parser.dart
+++ b/pkg/analyzer/lib/src/generated/parser.dart
@@ -5189,9 +5189,10 @@ class Parser {
       _reportErrorForToken(ParserErrorCode.EXPECTED_EXECUTABLE, _currentToken);
       return null;
     } else if (_tokenMatches(_peek(), TokenType.OPEN_PAREN)) {
+      TypeName returnType = _parseOptionalTypeNameComment();
       _validateModifiersForTopLevelFunction(modifiers);
       return _parseFunctionDeclaration(
-          commentAndMetadata, modifiers.externalKeyword, null);
+          commentAndMetadata, modifiers.externalKeyword, returnType);
     } else if (_peek()
         .matchesAny([TokenType.EQ, TokenType.COMMA, TokenType.SEMICOLON])) {
       if (modifiers.constKeyword == null &&