What have changed in the way of importing libraries?

I've implemented a method to read CSV files. On Sunday, it was working pretty fine. Today I receive the following error every time.

The CsvToListConverter was ok, then today... =/ Has something changed? The way of using libraries maybe?

--Talking for the user: he can select the file, then he gets an error. It is the same file I used to test this.

Line where I use the method:

String csvString = String.fromCharCodes(result.files.first.bytes!);
    CsvToListConverter converter = CsvToListConverter(
      fieldDelimiter: divider ?? ',',
    );