report const class as a more specific error code

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

Index: pkg/front_end/lib/src/fasta/parser/parser.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/parser.dart b/pkg/front_end/lib/src/fasta/parser/parser.dart
index 69bf837c23c75ef64a0bdec849ce2daca7158268..f1b75c63adddaa2913269020d67c07f93a1dcae8 100644
--- a/pkg/front_end/lib/src/fasta/parser/parser.dart
+++ b/pkg/front_end/lib/src/fasta/parser/parser.dart
@@ -286,12 +286,10 @@ class Parser {
             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);
         }