Fixed test analysis by fully implementing Collection members

Error was:

file:/html5lib.dart/lib/dom.dart:608:7: Concrete class FilteredElementList has unimplemented member(s) 
    # From Collection:
        void remove(Object)
   607: // I fixed this to extend Collection and implement removeAt and first.
   608: class FilteredElementList extends Collection<Element> implements List<Element> {