switch html5lib to new pkg layout

lib/src/list_proxy.dart

 // TODO(jmesserly): remove this once we have a subclassable growable list
 // in our libraries.
 
 /** A [List] proxy that you can subclass. */
 #library('list_proxy');
 
 /**
  * A [List<T>] proxy that you can subclass.
  */
 class ListProxy<T> implements List<T> {
@@ skipping 36 matching lines @@
   void clear() { _list.clear(); }
   T removeLast() => _list.removeLast();
   void setRange(int start, int length, List<T> from, [int startFrom]) {
     _list.setRange(start, length, from, startFrom);
   }
   void removeRange(int start, int length) { _list.removeRange(start, length); }
   void insertRange(int start, int length, [T initialValue]) {
     _list.insertRange(start, length, initialValue);
   }
 }