anchor.diff:  // BSD-style license that can be found in the LICENSE file.
   
   library fasta.stack_listener;
  -
  -import 'dart:collection' show Queue;
   
   import 'package:front_end/src/fasta/parser.dart' show ErrorKind, Listener;
   
  
positive.code: }
  
  abstract class StackListener extends Listener {
    final Queue<Object> stack = new Queue<Object>();
  
    Uri get uri;
  
  
negative.code: 
    List popList(int n) {
      if (n == 0) return null;
      return stack.popList(n);
    }
  
    void debugEvent(String name) {
  