Emojis don't get converted properly by the punycode algorithm

[bivens-dev]

Here is the failing test case:

test('With emoji', () {
  expect(punycodeEncoder.toAscii('\uD83D\uDCA9.la'), 'xn--ls8h.la');
});

and here is the error message:

 Expected: 'xn--ls8h.la'
    Actual: 'xn--8c9by4f.la'
     Which: is different.
            Expected: xn--ls8h.la
              Actual: xn--8c9by4f.la ...
                          ^
             Differ at offset 4
  
  package:matcher                        expect
  test/punycode/encoder_test.dart 194:9  main.<fn>.<fn>.<fn>