non_sync_factory

Factory bodies can't use 'async', 'async*', or 'sync*'.

The analyzer produces this diagnostic when the body of a factory constructor is marked with async, async*, or sync*. All constructors, including factory constructors, are required to return an instance of the class in which they're declared, not a Future, Stream, or Iterator.

The following code produces this diagnostic because the body of the factory constructor is marked with async:

class C {
  factory C() async {
    return C._();
  }
  C._();
}

If the member must be declared as a factory constructor, then remove the keyword appearing before the body:

class C {
  factory C() {
    return C._();
  }
  C._();
}

If the member must return something other than an instance of the enclosing class, then make the member a static method:

class C {
  static Future<C> m() async {
    return C._();
  }
  C._();
}