Consider supporting `Set`s in constructors

[jakemac53]

Consider a class like:

class MyThing {
  final Set<int> myInts;

  MyThing(this.myInts);
}

Today the generated fromJson constructor would end up trying to pass in an Iterable<int> to the constructor, which won't work. The workaround is:

class MyThing {
  final Set<int> myInts;

  MyThing(Iterable<int> myInts) : this.myInts = myInts.toSet();
}

...but ideally that wouldn't be necessary :).

fwiw, for List fields it will automatically call toList on the iterable before passing it in.

[kevmoo]

Reasonable. PRs welcome!