report const class as a more specific error code

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 '../fasta_codes.dart' show Code, Message, Template;
 
 import '../fasta_codes.dart' as fasta;
 
@@ skipping 268 matching lines @@
       token = token.next;
     }
     if (token.isTopLevelKeyword) {
       Token abstractToken;
       Token modifierToken = start;
       while (modifierToken != token) {
         if (optional('abstract', modifierToken) &&
             optional('class', token) &&
             abstractToken == null) {
           abstractToken = modifierToken;
+        } else if (optional('const', modifierToken) &&
+            optional('class', token)) {
+          reportRecoverableError(modifierToken, fasta.messageConstClass);
         } else {
-          // TODO(danrubel): Report more specific recoverable error message
-          // for `const class` to better help the user.
-          // See ParserErrorCode CONST_CLASS
-
-          // Report an error for each extraneous modifier
           reportRecoverableErrorWithToken(
               modifierToken, fasta.templateExtraneousModifier);
         }
         modifierToken = modifierToken.next;
       }
       return parseTopLevelKeywordDeclaration(abstractToken, token);
     } else if (token.isIdentifier || token.keyword != null) {
       // TODO(danrubel): improve parseTopLevelMember
       // so that we don't parse modifiers twice.
       return parseTopLevelMember(start);
@@ skipping 3878 matching lines @@
   }
 
   Token reportUnexpectedToken(Token token) {
     return reportUnrecoverableErrorWithToken(
         token, fasta.templateUnexpectedToken);
   }
 }
 
 // TODO(ahe): Remove when analyzer supports generalized function syntax.
 typedef _MessageWithArgument<T> = Message Function(T);