Upgrade to unittest 0.5.5
Update library to use Dart 0.5.5 APIs

- Use Iterable in place of Collection.
- Replace missing collection API methods, i.e. removeAll(), retainAll().
- Use if/else in place of switch statements when evaluating non-constants.
- Replace usage of List.getRange() with List.skip().take().

All tests are passing.
Merge pull request #26 from danschultz/upgrade_to_dart_0.5.5

Upgrade to Dart 0.5.5