Initial compression dictionary.
It should consist of strings (byte sequences) that are likely to be encountered later in the data to be compressed, with the most commonly used strings preferably put towards the end of the dictionary. Using a dictionary is most useful when the data to be compressed is short and can be predicted with good accuracy; the data can then be compressed better than with the default empty dictionary.
final List<int> dictionary
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https://api.dart.dev/stable/2.5.0/dart-io/GZipCodec/dictionary.html