switch html5lib to new pkg layout

lib/src/utils.dart

 /** Misc things that were useful when porting the code from Python. */
 #library('utils');
 
 #import('dart:math');
 #import('package:unittest/unittest.dart');
 #import('constants.dart');
 
 typedef bool Predicate();
 
 class Pair<F extends Hashable, S extends Hashable> implements Hashable {
@@ skipping 170 matching lines @@
       return new _ReverseIterable(_list).iterator();
     }
   }
 
   bool hasNext() => _index > 0;
   next() {
     if (_index == 0) throw const NoMoreElementsException();
     return _list[--_index];
   }
 }