Метод isDirectorySync
bool isDirectorySync(Синхронно проверяет, возвращает ли typeSync(path) FileSystemEntityType.DIRECTORY.
Исходный код
static bool isDirectorySync(String path) =>
(_getTypeSync(path, true) == FileSystemEntityType.DIRECTORY._type);
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https://api.dartlang.org/stable/1.24.3/dart-io/FileSystemEntity/isDirectorySync.html