Add support for material agnostic driver navigation (#22461)

examples/flutter_gallery/lib/demo/cupertino/cupertino_activity_indicator_demo.dart

@@ -13,7 +13,7 @@ class CupertinoProgressIndicatorDemo extends StatelessWidget {
   Widget build(BuildContext context) {
     return CupertinoPageScaffold(
       navigationBar: CupertinoNavigationBar(
-        previousPageTitle: 'Cupertino',
+        previousPageTitle: 'Back',
         middle: const Text('Cupertino Activity Indicator'),
         trailing: CupertinoDemoDocumentationButton(routeName),
       ),

examples/flutter_gallery/test_driver/transitions_perf_test.dart

@@ -45,9 +45,7 @@ const List<String> kUnsynchronizedDemos = <String>[
   'Video@Media',
 ];
 
-const List<String> kSkippedDemos = <String>[
-  'Pull to refresh@Cupertino', // The back button lacks a tooltip.
-];
+const List<String> kSkippedDemos = <String>[];
 
 // All of the gallery demos, identified as "title@category".
 //
@@ -157,10 +155,10 @@ Future<Null> runDemos(List<String> demos, FlutterDriver driver) async {
 
       if (kUnsynchronizedDemos.contains(demo)) {
         await driver.runUnsynchronized<void>(() async {
-          await driver.tap(find.byTooltip('Back'));
+          await driver.tap(find.pageBack());
         });
       } else {
-        await driver.tap(find.byTooltip('Back'));
+        await driver.tap(find.pageBack());
       }
     }
 

packages/flutter_driver/lib/src/common/find.dart

@@ -136,6 +136,7 @@ abstract class SerializableFinder {
       case 'ByValueKey': return ByValueKey.deserialize(json);
       case 'ByTooltipMessage': return ByTooltipMessage.deserialize(json);
       case 'ByText': return ByText.deserialize(json);
+      case 'PageBack': return PageBack();
     }
     throw DriverError('Unsupported search specification type $finderType');
   }
@@ -253,6 +254,17 @@ class ByType extends SerializableFinder {
   }
 }
 
+/// A Flutter Driver finder that finds the back button on the page's Material
+/// or Cupertino scaffold.
+///
+/// See also:
+///
+///  * [WidgetTester.pageBack], for a similar functionality in widget tests.
+class PageBack extends SerializableFinder {
+  @override
+  String get finderType => 'PageBack';
+}
+
 /// A Flutter driver command that retrieves a semantics id using a specified finder.
 ///
 /// This command requires assertions to be enabled on the device.

packages/flutter_driver/lib/src/driver/driver.dart

@@ -854,4 +854,7 @@ class CommonFinders {
 
   /// Finds widgets whose class name matches the given string.
   SerializableFinder byType(String type) => ByType(type);
+
+  /// Finds the back button on a Material or Cupertino page's scaffold.
+  SerializableFinder pageBack() => PageBack();
 }

packages/flutter_driver/lib/src/extension/extension.dart

@@ -7,6 +7,7 @@ import 'dart:async';
 import 'package:flutter/semantics.dart';
 import 'package:meta/meta.dart';
 
+import 'package:flutter/cupertino.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
@@ -138,6 +139,7 @@ class FlutterDriverExtension {
       'ByTooltipMessage': (SerializableFinder finder) => _createByTooltipMessageFinder(finder),
       'ByValueKey': (SerializableFinder finder) => _createByValueKeyFinder(finder),
       'ByType': (SerializableFinder finder) => _createByTypeFinder(finder),
+      'PageBack': (SerializableFinder finder) => _createPageBackFinder(),
     });
   }
 
@@ -274,6 +276,17 @@ class FlutterDriverExtension {
     }, description: 'widget with runtimeType "${arguments.type}"');
   }
 
+  Finder _createPageBackFinder() {
+    return find.byElementPredicate((Element element) {
+      final Widget widget = element.widget;
+      if (widget is Tooltip)
+        return widget.message == 'Back';
+      if (widget is CupertinoNavigationBarBackButton)
+        return true;
+      return false;
+    }, description: 'Material or Cupertino back button');
+  }
+
   Finder _createFinder(SerializableFinder finder) {
     final FinderConstructor constructor = _finders[finder.finderType];