title: How to Delete all documents in collection in Firestore with Flutter
body: <p>I have a firestore database. My project plugins:</p>
  
  <p>cloud_firestore: ^0.7.4
    firebase_storage: ^1.0.1</p>
  
  <p>This have a collection "messages" with a multiple documents.
  I need to delete all documents in the messages collection. But this code fail:</p>
  
  <pre><code>Firestore.instance.collection('messages').delete();
  </code></pre>
  
  <p>but delete is not define</p>
  
  <p>how is the correct syntax?</p>