pkg/third_party/html5lib: lots of cleanup

pkg/third_party/html5lib/lib/dom.dart

 /**
  * A simple tree API that results from parsing html. Intended to be compatible
  * with dart:html, but right now it resembles the classic JS DOM.
  */
 library dom;
 
 import 'dart:collection';
 import 'package:source_maps/span.dart' show FileSpan;
 
 import 'src/constants.dart';
 import 'src/list_proxy.dart';
 import 'src/token.dart';
 import 'src/tokenizer.dart';
-import 'src/treebuilder.dart';
 import 'src/utils.dart';
 import 'dom_parsing.dart';
 import 'parser.dart';
 
 // TODO(jmesserly): this needs to be replaced by an AttributeMap for attributes
 // that exposes namespace info.
 class AttributeName implements Comparable {
   /** The namespace prefix, e.g. `xlink`. */
   final String prefix;
 
@@ skipping 839 matching lines @@
     if (start == null) start = length - 1;
     return _filtered.lastIndexOf(element, start);
   }
 
   Element get first => _filtered.first;
 
   Element get last => _filtered.last;
 
   Element get single => _filtered.single;
 }