There's no built-in way to do this in Dart, but you can achieve it by using the rawMode property of the stdin object:
import 'dart:io';
main() {
stdout.write('Enter password: ');
stdin.rawMode = true; // disable input echo
var password = stdin.readLineSync();
stdin.rawMode = false; // re-enable input echo
stdout.writeln('You entered: $password');
}