unicodeReplacementCharacterRune верхнеуровневая константа
Символ замены Unicode U+FFFD (�).
Реализация
const int unicodeReplacementCharacterRune = 0xFFFD;
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Licensed under the BSD 3-Clause "New" or "Revised" License.
https://api.dart.dev/stable/2.19.6/dart-convert/unicodeReplacementCharacterRune-constant.html
