anchor.diff:  class FutureOr<T> {}
   
   abstract class Completer<T> {
  -  factory Completer() => new _AsyncCompleter<T>();
  -  factory Completer.sync() => new _SyncCompleter<T>();
  +  factory Completer() => null;
  +  factory Completer.sync() => null;
     Future<T> get future;
     void complete([value]);
     void completeError(Object error, [StackTrace stackTrace]);
  
positive.code:   Iterable/*<R>*/ map/*<R>*/(/*=R*/ f(E e));
  
    /*=R*/ fold/*<R>*/(/*=R*/ initialValue,
        /*=R*/ combine(/*=R*/ previousValue, E element));
  
    Iterable/*<T>*/ expand/*<T>*/(Iterable/*<T>*/ f(E element));
  
  
negative.code: import 'dart:async';
  
  class Object {
    const Object();
    bool operator ==(other) => identical(this, other);
    String toString() => 'a string';
    int get hashCode => 0;
  