fix script tags -- remove special case handling for polymer-element

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';
@@ skipping 163 matching lines @@
     return str.toString();
   }
 
   set innerHtml(String value) {
     nodes.clear();
     // TODO(jmesserly): should be able to get the same effect by adding the
     // fragment directly.
     nodes.addAll(parseFragment(value, container: tagName).nodes);
   }
 
+  Node get firstChild => nodes.isNotEmpty ? nodes[0] : null;
+
   void _addOuterHtml(StringBuffer str);
 
   void _addInnerHtml(StringBuffer str) {
     for (Node child in nodes) child._addOuterHtml(str);
   }
 
   String toString() => tagName;
 
   Node remove() {
     // TODO(jmesserly): is parent == null an error?
@@ skipping 668 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;
 }