Add a basic scrolling test for the Gallery home page.

Current numbers from my Nexus 5:
  "average_frame_build_time_millis": 22.6854,
  "missed_frame_build_budget_count": 15,
  "frame_count": 15

Filed https://github.com/flutter/flutter/issues/3296

@yjbanov @hansmuller @vlidholt

examples/material_gallery/pubspec.yaml

@@ -16,3 +16,5 @@ dev_dependencies:
   test: any # flutter_test provides the version constraints
   flutter_test:
     path: ../../packages/flutter_test
+  flutter_driver:
+    path: ../../packages/flutter_driver

examples/material_gallery/test_driver/scroll_perf.dart

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_driver/driver_extension.dart';
+import 'package:material_gallery/main.dart' as app;
+
+void main() {
+  enableFlutterDriverExtension();
+  app.main();
+}

examples/material_gallery/test_driver/scroll_perf_test.dart

+// Copyright 2016 The Chromium Authors. 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:async';
+import 'package:flutter_driver/flutter_driver.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('scrolling performance test', () {
+    FlutterDriver driver;
+
+    setUpAll(() async {
+      driver = await FlutterDriver.connect();
+    });
+
+    tearDownAll(() async {
+      if (driver != null)
+        driver.close();
+    });
+
+    test('measure', () async {
+      Timeline timeline = await driver.traceAction(() async {
+        // Find the scrollable stock list
+        ObjectRef stockList = await driver.findByValueKey('Gallery List');
+        expect(stockList, isNotNull);
+
+        await driver.tap(await driver.findByText('Demos'));
+        await driver.tap(await driver.findByText('Components'));
+        await driver.tap(await driver.findByText('Style'));
+
+        // Scroll down
+        for (int i = 0; i < 5; i++) {
+          await driver.scroll(stockList, 0.0, -300.0, new Duration(milliseconds: 300));
+          await new Future<Null>.delayed(new Duration(milliseconds: 500));
+        }
+
+        // Scroll up
+        for (int i = 0; i < 5; i++) {
+          await driver.scroll(stockList, 0.0, 300.0, new Duration(milliseconds: 300));
+          await new Future<Null>.delayed(new Duration(milliseconds: 500));
+        }
+      });
+
+      TimelineSummary summary = new TimelineSummary.summarize(timeline);
+      summary.writeSummaryToFile('home_scroll_perf', pretty: true);
+      summary.writeTimelineToFile('home_scroll_perf', pretty: true);
+    });
+  });
+}