Change the calling conventions for handleUnrecoverableError.

pkg/front_end/lib/src/fasta/parser/parser.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.parser.parser;
 
 import '../scanner.dart' show ErrorToken;
 
 import '../scanner/recover.dart' show closeBraceFor, skipToEof;
 
@@ skipping 3507 matching lines @@
 
   /// Don't call this method. Should only be used as a last resort when there
   /// is no feasible way to recover from a parser error.
   Token reportUnrecoverableError(Token token, ErrorKind kind, [Map arguments]) {
     Token next;
     if (token is ErrorToken) {
       next = reportErrorToken(token, false);
     } else {
       arguments ??= {};
       arguments.putIfAbsent("actual", () => token.value);
-      next = listener.handleUnrecoverableError(token, kind, arguments);
+      next = listener.handleUnrecoverableError(token, kind, arguments)?.next;
     }
     return next ?? skipToEof(token);
   }
 
   void reportRecoverableError(Token token, ErrorKind kind, [Map arguments]) {
     if (token is ErrorToken) {
       reportErrorToken(token, true);
     } else {
       arguments ??= {};
       listener.handleRecoverableError(token, kind, arguments);
@@ skipping 30 matching lines @@
         arguments = {"text": token.assertionMessage};
         break;
 
       default:
         break;
     }
     if (isRecoverable) {
       listener.handleRecoverableError(token, kind, arguments);
       return null;
     } else {
-      return listener.handleUnrecoverableError(token, kind, arguments);
+      return listener.handleUnrecoverableError(token, kind, arguments)?.next;
     }
   }
 }