Create more convenience methods for Firestore (flutter#308)

* Create an `add` method for CollectionReference.

This convenience method helps the Flutter API match Firebase's Android API. Based on user feedback.

* nit - remove a space

* Match DocumentSnapshot's documentID method for convenience.

* Typo in docs

* Double the typo double the fun

* Update pubspec.yaml

* Update CHANGELOG.md

Author: Kyle Bradshaw

packages/cloud_firestore/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 0.2.4
+
+* Support for DocumentReference.documentID
+* Support for CollectionReference.add
+
 ## 0.2.3
 
 * Simplified and upgraded Android project template to Android SDK 27.

packages/cloud_firestore/lib/src/collection_reference.dart

@@ -26,7 +26,7 @@ class CollectionReference extends Query {
   ///
   /// If no [path] is provided, an auto-generated ID is used.
   ///
-  /// The unique key generated by is prefixed with a client-generated timestamp
+  /// The unique key generated is prefixed with a client-generated timestamp
   /// so that the resulting list will be chronologically-sorted.
   DocumentReference document([String path]) {
     List<String> childPath;
@@ -39,6 +39,17 @@ class CollectionReference extends Query {
     }
     return new DocumentReference._(_firestore, childPath);
   }
+
+  /// Returns a `DocumentReference` with an auto-generated ID, after
+  /// populating it with provided [data].
+  ///
+  /// The unique key generated is prefixed with a client-generated timestamp
+  /// so that the resulting list will be chronologically-sorted.
+  Future<DocumentReference> add(Map<String, dynamic> data) async {
+    final DocumentReference newDocument = document();
+    await newDocument.setData(data);
+    return newDocument;
+  }
 }
 
 class ServerValue {

packages/cloud_firestore/lib/src/document_reference.dart

@@ -22,6 +22,9 @@ class DocumentReference {
   /// Slash-delimited path representing the database location of this query.
   String get path => _pathComponents.join('/');
 
+  /// This document's given or generated ID in the collection.
+  String get documentID => _pathComponents.last;
+
   /// Writes to the document referred to by this [DocumentReference]. If the
   /// document does not yet exist, it will be created. If you pass [SetOptions],
   /// the provided data will be merged into an existing document.

packages/cloud_firestore/pubspec.yaml

@@ -3,7 +3,7 @@ description: Flutter plugin for Cloud Firestore, a cloud-hosted, noSQL database
   live synchronization and offline support on Android and iOS.
 author: Flutter Team <flutter-dev@googlegroups.com>
 homepage: https://github.com/flutter/plugins/tree/master/packages/cloud_firestore
-version: 0.2.3
+version: 0.2.4
 
 flutter:
   plugin: