feat(firestore): count() feature for counting documents without retrieving documents.

packages/cloud_firestore/cloud_firestore/example/ios/Podfile

@@ -56,6 +56,9 @@ end
 
 post_install do |installer|
   installer.pods_project.targets.each do |target|
+    target.build_configurations.each do |config|
+      config.build_settings.delete 'IPHONEOS_DEPLOYMENT_TARGET'
+    end
     flutter_additional_ios_build_settings(target)
   end
 end

packages/cloud_firestore/cloud_firestore/example/test_driver/query_e2e.dart

@@ -7,6 +7,7 @@ import 'dart:math';
 
 import 'package:cloud_firestore/cloud_firestore.dart';
 import 'package:firebase_core/firebase_core.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void runQueryTests() {
@@ -1807,6 +1808,29 @@ void runQueryTests() {
         },
         timeout: const Timeout.factor(3),
       );
+
+      test(
+        'count()',
+        () async {
+          final collection = await initializeTest('count');
+
+          await Future.wait([
+            collection.add({'foo': 'bar'}),
+            collection.add({'bar': 'baz'})
+          ]);
+
+          AggregateQuery query = collection.count();
+
+          AggregateQuerySnapshot snapshot = await query.get();
+
+          expect(
+            snapshot.count,
+            2,
+          );
+          // TODO(russellwheatley): remove when android code implemented
+        },
+        skip: defaultTargetPlatform == TargetPlatform.android,
+      );
     });
   });
 }