Update xml_element.dart

changed addChildren from addChildren(Collection<XmlNode> elements) to 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 ... 
Sorry but I am new in this ...
Merge pull request #22 from nickfloros/master

attempt to fix issue 21