From the Firestore docs:

If the document does exist, its contents will be overwritten with the newly provided data, unless you specify that the data should be merged into the existing document, as follows:

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

var cityRef = db.collection('cities').doc('BJ');

var setWithMerge = cityRef.set({
    capital: true
}, { merge: true });

Please add this feature to Flutter's Firestore plugin.