attempt to fix issue 21

changed addChildren from addChildren(Collection<XmlNode> elements) to addChildren(List<XmlNode> elements) to address https://github.com/prujohn/dart-xml/issues/21. 

however attempt to run the unit tests through dart m4 threw a number of other faults in 
Unhandled exception:
'package:unittest/unittest.dart': Error: line 788 pos 28: type 'ExpectException' is not loaded
    String message = (e is ExpectException) ? e.message : 'Caught $e';

I have no idea what is happening ... as I rather new to all this ...