[html5lib] implement querySelector/querySelectorAll

pkg/third_party/html5lib/lib/src/constants.dart

 library constants;
 
 import 'utils.dart';
 
 // TODO(jmesserly): fix up the const lists. For the bigger ones, we need faster
 // lookup than linear search "contains". In the Python code they were
 // frozensets.
 
 final String EOF = null;
 
@@ skipping 298 matching lines @@
   Namespaces._();
 
   static String getPrefix(String url) {
     switch (url) {
       case html: return 'html';
       case mathml: return 'math';
       case svg: return 'svg';
       case xlink: return 'xlink';
       case xml: return 'xml';
       case xmlns: return 'xmlns';
-      default: throw new ArgumentError(url);
+      default: return null;
     }
   }
 }
 
 const List scopingElements = const [
   const Pair(Namespaces.html, "applet"),
   const Pair(Namespaces.html, "caption"),
   const Pair(Namespaces.html, "html"),
   const Pair(Namespaces.html, "marquee"),
   const Pair(Namespaces.html, "object"),
@@ skipping 2783 matching lines @@
   'windows1252': 'cp1252',
   'windows1253': 'cp1253',
   'windows1254': 'cp1254',
   'windows1255': 'cp1255',
   'windows1256': 'cp1256',
   'windows1257': 'cp1257',
   'windows1258': 'cp1258',
   'windows936': 'gbk',
   'x-x-big5': 'big5'
 };