anchor.diff:  
     @override
     Expression deprecated_buildCompileTimeError(String error,
  -      [int charOffset = -1]) {
  -    return new SyntheticExpressionJudgment(buildCompileTimeError(
  -        fasta.templateUnspecified.withArguments(error), charOffset, noLength));
  +      [int charOffset = -1, fasta.Message message]) {
  +    message ??= fasta.templateUnspecified.withArguments(error);
  +    return new SyntheticExpressionJudgment(
  +        buildCompileTimeError(message, charOffset, noLength));
     }
   
     @override
  
positive.code:       push(forest.block(
            token,
            <Statement>[
              deprecated_buildCompileTimeErrorStatement(
                  "Expected '{'.", token.charOffset)
            ],
            null));
      }
  
negative.code:   }
  
    Statement deprecated_buildCompileTimeErrorStatement(error,
        [int charOffset = -1, fasta.Message message]) {
      return new ExpressionStatementJudgment(
          deprecated_buildCompileTimeError(error, charOffset, message), null);
    }
  
    Statement buildCompileTimeErrorStatement(Message message, int charOffset,
  