Support Firestore.runTransaction

[suztomo]

(I haven’t tested but I assume the function is not implemented yet. So far I don’t use transactions in my Flutter app)

https://firebase.google.com/docs/firestore/manage-data/transactions#transactions

final DocumentReference postRef = Firestore.instance.document('posts/123');
Firestore.instance.runTransaction((Transaction tx) async {
  DocumentSnapshot postSnapshot = await tx.get(postRef);
  if (postSnapshot.exists) {
    await tx.update(postRef, <String, dynamic>{'likesCount': postSnapshot.data['likesCount'] + 1});
  }
});

Excerpt from https://pub.dev/packages/cloud_firestore