Make dart-uuid strong-mode clean.

[harryterkelsen]

@daegalus

[daegalus]

Thank you very much for these changes. I will confirm everything works still, and publish a new version as soon as I get home in a few hours.

[daegalus]

$ dart test/uuid_test.dart
00:00 �+0�: [Version 1 Tests] IDs created at same mSec are different�
00:00 �+1�: [Version 1 Tests] Exception thrown when > 10K ids created in 1 ms�
00:00 �+2�: [Version 1 Tests] Clock regression by msec increments the clockseq - mSec�
00:00 �+3�: [Version 1 Tests] Clock regression by msec increments the clockseq - nSec�
00:00 �+4�: [Version 1 Tests] Explicit options produce expected id�
00:00 �+5�: [Version 1 Tests] Ids spanning 1ms boundary are 100ns apart�
00:00 �+6�: [Version 4 Tests] Check if V4 is consistent using a static seed�
00:00 �+7�: [Version 4 Tests] Return same output as entered for "random" option�
00:00 �+8�: [Version 4 Tests] Make sure that really fast uuid.v4 doesn't produce duplicates�
00:00 �+9�: [Version 5 Tests] Using URL namespace and custom name�
'package:typed_data/typed_buffers.dart': error: line 175 pos 30: unexpected token '?'
    if (values is List) end ??= values.length;

Seems there is an error. I'll fix it when i get the chance unless you want to make a new pull request (I use drone for testing, and I haven't setup pull request testing yet, so I had to merge to see tests run)