operator [] method
The character associated with a particular byte in this code page.
The byte
must be in the range 0..255.
The returned value should be a Unicode scalar value
(a non-surrogate code point).
If a code page does not have a defined character for a particular byte, it should return the Unicode invalid character (U+FFFD) instad.
Implementation
int operator [](int byte) => decoder._char(byte);