pkg/third_party/html5lib: lots of cleanup

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

 /** Misc things that were useful when porting the code from Python. */
 library utils;
 
-import 'dart:collection';
 import 'constants.dart';
 
 typedef bool Predicate();
 
 class Pair<F, S> {
   final F first;
   final S second;
 
   const Pair(this.first, this.second);
 
@@ skipping 100 matching lines @@
 
       last = match + 1;
     }
 
     result.write(format.substring(last, format.length));
     format = result.toString();
   });
 
   return format;
 }