Latin1Decoder constructor
Instantiates a new Latin1Decoder.
The optional allowInvalid argument defines how convert deals
with invalid bytes.
If it is true, convert replaces invalid bytes with the Unicode
Replacement character U+FFFD (�).
Otherwise it throws a FormatException.
Implementation
const Latin1Decoder({bool allowInvalid: false})
    : super(allowInvalid, _latin1Mask);