Add a way to generically go back on both Material and Cupertino scaffold pages (#15602)

* Add a way to generically go back on both Material and Cupertino scaffold pages

* review

* lint

* review

* Fix text assert text

packages/flutter_test/lib/src/widget_tester.dart

@@ -4,6 +4,7 @@
 
 import 'dart:async';
 
+import 'package:flutter/cupertino.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
@@ -15,6 +16,7 @@ import 'all_elements.dart';
 import 'binding.dart';
 import 'controller.dart';
 import 'finders.dart';
+import 'matchers.dart';
 import 'test_async_utils.dart';
 import 'test_text_input.dart';
 
@@ -531,6 +533,21 @@ class WidgetTester extends WidgetController implements HitTestDispatcher, Ticker
       await idle();
     });
   }
+
+  /// Makes an effort to dismiss the current page with a Material [Scaffold] or
+  /// a [CupertinoPageScaffold].
+  ///
+  /// Will throw an error if there is no back button in the page.
+  Future<void> pageBack() async {
+    Finder backButton = find.byTooltip('Back');
+    if (backButton.evaluate().isEmpty) {
+      backButton = find.widgetWithIcon(CupertinoButton, CupertinoIcons.back);
+    }
+
+    expect(backButton, findsOneWidget, reason: 'One back button expected on screen');
+
+    await tap(backButton);
+  }
 }
 
 typedef void _TickerDisposeCallback(_TestTicker ticker);

packages/flutter_test/test/widget_tester_test.dart

@@ -2,7 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter/widgets.dart';
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 const List<Widget> fooBarTexts = const <Text>[
@@ -195,7 +196,9 @@ void main() {
       expect(failure, isNotNull);
       expect(
         failure.message,
-        contains('Actual: ?:<zero widgets with text "bar" that has ancestor(s) with type Column with text "foo"')
+        contains(
+          'Actual: ?:<zero widgets with text "bar" that has ancestor(s) with type "Column" which is an ancestor of text "foo"',
+        ),
       );
     });
   });
@@ -255,7 +258,9 @@ void main() {
       expect(failure, isNotNull);
       expect(
         failure.message,
-        contains('Actual: ?:<zero widgets with type Column with text "foo" which is an ancestor of text "bar"'),
+        contains(
+          'Actual: ?:<zero widgets with type "Column" which is an ancestor of text "foo" which is an ancestor of text "bar"',
+        ),
       );
     });
 
@@ -289,6 +294,94 @@ void main() {
     });
   });
 
+  group('pageBack', (){
+    testWidgets('fails when there are no back buttons', (WidgetTester tester) async {
+      await tester.pumpWidget(new Container());
+
+      expect(
+        expectAsync0(tester.pageBack),
+        throwsA(const isInstanceOf<TestFailure>()),
+      );
+    });
+
+    testWidgets('successfully taps material back buttons', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        new MaterialApp(
+          home: new Center(
+            child: new Builder(
+              builder: (BuildContext context) {
+                return new RaisedButton(
+                  child: const Text('Next'),
+                  onPressed: () {
+                    Navigator.push<void>(context, new MaterialPageRoute<void>(
+                      builder: (BuildContext context) {
+                        return new Scaffold(
+                          appBar: new AppBar(
+                            title: const Text('Page 2'),
+                          ),
+                        );
+                      },
+                    ));
+                  },
+                );
+              } ,
+            ),
+          ),
+        ),
+      );
+
+      await tester.tap(find.text('Next'));
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 400));
+
+      await tester.pageBack();
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 400));
+
+      expect(find.text('Next'), findsOneWidget);
+      expect(find.text('Page 2'), findsNothing);
+    });
+
+    testWidgets('successfully taps cupertino back buttons', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        new MaterialApp(
+          home: new Center(
+            child: new Builder(
+              builder: (BuildContext context) {
+                return new CupertinoButton(
+                  child: const Text('Next'),
+                  onPressed: () {
+                    Navigator.push<void>(context, new CupertinoPageRoute<void>(
+                      builder: (BuildContext context) {
+                        return new CupertinoPageScaffold(
+                          navigationBar: const CupertinoNavigationBar(
+                            middle: const Text('Page 2'),
+                          ),
+                          child: new Container(),
+                        );
+                      },
+                    ));
+                  },
+                );
+              } ,
+            ),
+          ),
+        ),
+      );
+
+      await tester.tap(find.text('Next'));
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 400));
+
+      await tester.pageBack();
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 400));
+
+      expect(find.text('Next'), findsOneWidget);
+      expect(find.text('Page 2'), findsNothing);
+    });
+  });
+
   testWidgets('hasRunningAnimations control test', (WidgetTester tester) async {
     final AnimationController controller = new AnimationController(
       duration: const Duration(seconds: 1),