Align the flutter_gallery test to the middle for better results (#8015)

examples/flutter_gallery/test_driver/transitions_perf_test.dart

@@ -11,14 +11,6 @@ import 'package:flutter_driver/flutter_driver.dart';
 import 'package:path/path.dart' as path;
 import 'package:test/test.dart';
 
-// Warning: this list must be kept in sync with the value of
-// kAllGalleryItems.map((GalleryItem item) => item.category)).toList();
-final List<String> demoCategories = <String>[
-  'Demos',
-  'Components',
-  'Style'
-];
-
 // Warning: this list must be kept in sync with the value of
 // kAllGalleryItems.map((GalleryItem item) => item.title).toList();
 final List<String> demoTitles = <String>[
@@ -148,17 +140,12 @@ void main() {
 
     test('all demos', () async {
       Timeline timeline = await driver.traceAction(() async {
-        // Expand the demo category submenus.
-        for (String category in demoCategories.reversed) {
-          await driver.tap(find.text(category));
-          await new Future<Null>.delayed(kWaitBetweenActions);
-        }
         // Scroll each demo menu item into view, launch the demo and
         // return to the demo menu 2x.
         for(String demoTitle in demoTitles) {
           print('Testing "$demoTitle" demo');
           SerializableFinder menuItem = find.text(demoTitle);
-          await driver.scrollIntoView(menuItem);
+          await driver.scrollIntoView(menuItem, alignment: 0.5);
           await new Future<Null>.delayed(kWaitBetweenActions);
 
           for(int i = 0; i < 2; i += 1) {

packages/flutter_driver/lib/src/driver.dart

@@ -357,8 +357,8 @@ class FlutterDriver {
 
   /// Scrolls the Scrollable ancestor of the widget located by [finder]
   /// until the widget is completely visible.
-  Future<Null> scrollIntoView(SerializableFinder finder) async {
-    return await _sendCommand(new ScrollIntoView(finder)).then((Map<String, dynamic> _) => null);
+  Future<Null> scrollIntoView(SerializableFinder finder, { double alignment: 0.0 }) async {
+    return await _sendCommand(new ScrollIntoView(finder, alignment: alignment)).then((Map<String, dynamic> _) => null);
   }
 
   /// Returns the text in the `Text` widget located by [finder].

packages/flutter_driver/lib/src/extension.dart

@@ -249,7 +249,7 @@ class _FlutterDriverExtension {
     ScrollIntoView scrollIntoViewCommand = command;
     Finder target = await _waitForElement(_createFinder(scrollIntoViewCommand.finder));
     await Scrollable.ensureVisible(target.evaluate().single, duration: const Duration(milliseconds: 100));
-    await Scrollable2.ensureVisible(target.evaluate().single, duration: const Duration(milliseconds: 100));
+    await Scrollable2.ensureVisible(target.evaluate().single, duration: const Duration(milliseconds: 100), alignment: scrollIntoViewCommand.alignment ?? 0.0);
     return new ScrollResult();
   }
 

packages/flutter_driver/lib/src/gesture.dart

@@ -95,14 +95,17 @@ class ScrollIntoView extends CommandWithTarget {
 
   /// Creates this command given a [finder] used to locate the widget to be
   /// scrolled into view.
-  ScrollIntoView(SerializableFinder finder) : super(finder);
+  ScrollIntoView(SerializableFinder finder, { this.alignment: 0.0 }) : super(finder);
 
   /// Deserializes this command from JSON generated by [serialize].
   ScrollIntoView.deserialize(Map<String, String> json)
-      : super.deserialize(json);
+      : this.alignment = double.parse(json['alignment']),
+        super.deserialize(json);
+
+  final double alignment;
 
-  // This is here just to be clear that this command isn't adding any extra
-  // fields.
   @override
-  Map<String, String> serialize() => super.serialize();
+  Map<String, String> serialize() => super.serialize()..addAll(<String, String>{
+    'alignment': '$alignment',
+  });
 }