anchor.diff:  }
   
   abstract class RegExp implements Pattern {
  -  external factory RegExp(String source);
  +  external factory RegExp(String source, {bool unicode = false});
   }
   
   abstract class Set<E> implements Iterable<E> {
  
positive.code: 
    Future<T> whenComplete(action());
  
    static Future<List<T>> wait<T>(Iterable<Future<T>> futures) => throw 0;
  }
  
  abstract class FutureOr<T> {}
  
negative.code:   }
  }
  
  abstract class IterableMixin<E> implements Iterable<E> { }
  
  abstract class LinkedHashMap<K, V> implements Map<K, V> {
    external factory LinkedHashMap(
        {bool Function(K, K)? equals,
  