anchor.diff:        case State.before:
           state = State.next;
           current = iterator.current;
  -        break;
  +        return true;
         case State.next:
           if (iterator.moveNext()) {
             state = State.separator;
  
positive.code: 
  import 'dart:collection';
  
  /// Function type to build separator elements.
  typedef Builder<E> = E Function();
  
  /// States of the iterator statemachine.
  
negative.code:       case State.separator:
          state = State.next;
          current = iterator.current;
          return true;
        case State.after:
          state = State.complete;
          current = null;
          return false;
        case State.complete:
          return false;
      }
  