invalid_modifier_on_constructor

The modifier '{0}' can't be applied to the body of a constructor.

The analyzer produces this diagnostic when the body of a constructor is prefixed by one of the following modifiers: async, async*, or sync*. Constructor bodies must be synchronous.

The following code produces this diagnostic because the body of the constructor for C is marked as being async:

class C {
  C() async {}
}

If the constructor can be synchronous, then remove the modifier:

class C {
  C();
}

If the constructor can't be synchronous, then use a static method to create the instance instead:

class C {
  C();
  static Future<C> c() async {
    return C();
  }
}