title: Flutter Firestore add new document with Custom ID
body: <p>How to add new document with custom id using Dart and Flutter?</p>
  <blockquote>
  <p>PS: I can add new document to collection but its id sets randomly, using this code</p>
  <p><code>postRef.add(data);</code></p>
  <p>which <code>postRef</code> is <code>CollectionReference</code> and <code>data</code> is <code>Map&lt;String, dynamic&gt;</code></p>
  </blockquote>