Restructure parser error handling and recovery.

pkg/fasta/lib/src/source/diet_parser.dart

 // Copyright (c) 2016, 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.diet_parser;
 
 import 'package:dart_scanner/src/token.dart' show
     BeginGroupToken,
     Token;
 
@@ skipping 13 matching lines @@
 class DietParser extends ClassMemberParser {
   DietParser(Listener listener, {bool asyncAwaitKeywordsEnabled: false})
       : super(listener, asyncAwaitKeywordsEnabled: asyncAwaitKeywordsEnabled);
 
   Token parseFormalParameters(Token token) => skipFormals(token);
 
   Token skipFormals(Token token) {
     listener.beginOptionalFormalParameters(token);
     if (!optional('(', token)) {
       if (optional(';', token)) {
-        listener.reportError(token, ErrorKind.ExpectedOpenParens);
+        reportRecoverableError(token, ErrorKind.ExpectedOpenParens, {});
         return token;
       }
-      return listener.unexpected(token);
+      return reportUnrecoverableError(token, ErrorKind.UnexpectedToken);
     }
     BeginGroupToken beginGroupToken = token;
     Token endToken = beginGroupToken.endGroup;
     listener.endFormalParameters(0, token, endToken);
     return endToken.next;
   }
 }