fix fasta scanner comment processing

pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart

Index: pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart b/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
index c6a92b94b26b5101d240618775007178e3782303..9b113e038660d0492e643e79d2cb8cc2e59ba402 100644
--- a/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
@@ -897,7 +897,7 @@ abstract class AbstractScanner implements Scanner {
     } else {
       // It is the responsibility of the caller to construct the token
       // being appended with preceeding comments if any
-      assert(comments == null || token.isSynthetic);
+      assert(comments == null || token.isSynthetic || token is ErrorToken);
     }
   }