Generated Dart code expects PbMap instead of Map which is hard to create

[gryznar]

Previously it was:

class Foo extends $pb.GeneratedMessage {
  factory Foo({
    $core.Map<$core.String, Bar>? bars,
  }) {
    final $result = create();
    if (bars!= null) {
      $result.bars.addAll(bars);
    }
    return $result;
  }
  ...
}

now it is:

class Foo extends $pb.GeneratedMessage {
  factory Foo({
    $pb.PbMap<$core.String, Bar>? bars,
  }) {
    final $result = create();
    if (bars!= null) {
      $result.bars.addAll(bars);
    }
    return $result;
  }
  ...
}

Creating PbMap manually is very problematic, as it expects 2 ints (one for key and one for value). Personally I don't know how to obtain them. Situation is much simpler with PbList which just exposes .from(List<E> from) constructor

[mraleph]

I think we should probably either under the #903 change partially or - if narrower type actually leads to some benefits - we should provide a convenient way to create these.

[osa1]

Good catch -- the factory argument type does not need to be a PbMap or even a Map, it could be any iterable that yields key-value pairs with the right types.

This change was done a while ago and I don't remember the details, but it must've been unintentional. I changed how the field is represented and that effected factory argument types as well. We can and should distinguish those two.

We can make the argument type Iterable<MapEntry<K, V>>.

For now, as a work around, you can create an empty message and then add the map elements with message.mapField.addAll or message.mapField[key] = value.