Upgrade to Dart 0.5.5

Updated the following:

* 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().
* Upgrade to unittest version 0.5.5.

Should fix issues: prujohn/dart-xml#21, prujohn/dart-xml#23.

All tests are passing.