Code cleanup to dart 17594

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>
Return a List<int> rather than a Map when doing toByteArray as this is more useful

In particular other libraries consider List<int> the way to represent a byte array, it might also have better performance

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>
Merge pull request #11 from ucam-cl-dtg/master

Fix to recent dart and return a List<int> rather than a map