Add separate driver file for performance tests (#890)

testing_app/README.md

@@ -30,7 +30,7 @@ The Flutter SDK can run unit tests and widget tests in a virtual machine, withou
 - Integration Tests:
   - Run `flutter drive --driver=integration_test/driver.dart --target=integration_test/app_test.dart`
 - Performance Tests:
-  - Run `flutter drive --driver=integration_test/driver.dart --target=integration_test/perf_test.dart --profile --trace-startup`
+  - Run `flutter drive --driver=integration_test/perf_driver.dart --target=integration_test/perf_test.dart --profile --trace-startup`
     - Using a physical device and running performance tests in profile mode is recommended.
     - The `--trace-startup` option is used to avoid flushing older timeline events when the timeline gets long.
 - State Management Tests:

testing_app/integration_test/perf_driver.dart

@@ -0,0 +1,25 @@
+// Copyright 2021 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:developer';
+
+import 'package:integration_test/integration_test_driver.dart';
+
+Future<void> main() {
+  return integrationDriver(
+    responseDataCallback: (data) async {
+      // If the tests reported any data, save it to the disk.
+      if (data != null) {
+        for (var entry in data.entries) {
+          log('Writing ${entry.key} to the disk.');
+          // Default storage destination is the 'build' directory.
+          await writeResponseData(
+            entry.value as Map<String, dynamic>,
+            testOutputFilename: entry.key,
+          );
+        }
+      }
+    },
+  );
+}