Support accessibility back gesture to pop route from view controller (#51241)

Adapted suggestion from flutter/flutter#74246 (comment) to pop the route if the view controller route if `accessibilityPerformEscape` is called at that level in the responder chain.

Ideally this could know if the route was successfully popped, and only then return `YES` (halting propagation through the responder chain).

I confirmed by editing https://docs.flutter.dev/cookbook/navigation/navigation-basics#interactive-example as a demo that that the two-finger scrub (move two fingers back and forth three times quickly, making a "z") navigates back to the last route, and that the [`SementicObject`](https://github.com/flutter/engine/blob/88882292c952d76069cee67d76acc0fbae51c749/shell/platform/darwin/ios/framework/Source/SemanticsObject.mm#L763) scrub, which already worked, continued to work.

I'm not super familiar with this code, hopefully there aren't further gotchas I'm not thinking of.

Fixes flutter/flutter#74246

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: jmagman

shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -2104,6 +2104,15 @@ - (int32_t)accessibilityFlags {
   return flags;
 }
 
+- (BOOL)accessibilityPerformEscape {
+  FlutterMethodChannel* navigationChannel = [_engine.get() navigationChannel];
+  if (navigationChannel) {
+    [self popRoute];
+    return YES;
+  }
+  return NO;
+}
+
 + (BOOL)accessibilityIsOnOffSwitchLabelsEnabled {
   if (@available(iOS 13, *)) {
     return UIAccessibilityIsOnOffSwitchLabelsEnabled();

shell/platform/darwin/ios/framework/Source/FlutterViewControllerTest.mm

@@ -1365,6 +1365,22 @@ - (void)testItReportsAccessibilityOnOffSwitchLabelsFlagSet {
   XCTAssert((flags & (int32_t)flutter::AccessibilityFeatureFlag::kOnOffSwitchLabels) != 0);
 }
 
+- (void)testAccessibilityPerformEscapePopsRoute {
+  FlutterEngine* mockEngine = OCMPartialMock([[FlutterEngine alloc] init]);
+  [mockEngine createShell:@"" libraryURI:@"" initialRoute:nil];
+  id mockNavigationChannel = OCMClassMock([FlutterMethodChannel class]);
+  OCMStub([mockEngine navigationChannel]).andReturn(mockNavigationChannel);
+
+  FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:mockEngine
+                                                                                nibName:nil
+                                                                                 bundle:nil];
+  XCTAssertTrue([viewController accessibilityPerformEscape]);
+
+  OCMVerify([mockNavigationChannel invokeMethod:@"popRoute" arguments:nil]);
+
+  [mockNavigationChannel stopMocking];
+}
+
 - (void)testPerformOrientationUpdateForcesOrientationChange {
   [self orientationTestWithOrientationUpdate:UIInterfaceOrientationMaskPortrait
                           currentOrientation:UIInterfaceOrientationLandscapeLeft

testing/ios/IosUnitTests/IosUnitTests.xcodeproj/project.pbxproj

@@ -50,7 +50,6 @@
 		0AC232F724BA71D300A85907 /* FlutterEngineTest.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = FlutterEngineTest.mm; sourceTree = "<group>"; };
 		0AC2330324BA71D300A85907 /* accessibility_bridge_test.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = accessibility_bridge_test.mm; sourceTree = "<group>"; };
 		0AC2330B24BA71D300A85907 /* FlutterTextInputPluginTest.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = FlutterTextInputPluginTest.mm; sourceTree = "<group>"; };
-		0AC2330F24BA71D300A85907 /* FlutterBinaryMessengerRelayTest.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = FlutterBinaryMessengerRelayTest.mm; sourceTree = "<group>"; };
 		0AC2331024BA71D300A85907 /* connection_collection_test.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = connection_collection_test.mm; sourceTree = "<group>"; };
 		0AC2331224BA71D300A85907 /* FlutterEnginePlatformViewTest.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = FlutterEnginePlatformViewTest.mm; sourceTree = "<group>"; };
 		0AC2331924BA71D300A85907 /* FlutterPluginAppLifeCycleDelegateTest.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = FlutterPluginAppLifeCycleDelegateTest.mm; sourceTree = "<group>"; };
@@ -72,7 +71,6 @@
 		3DD7D38C27D2B81000DA365C /* FlutterUndoManagerPluginTest.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = FlutterUndoManagerPluginTest.mm; sourceTree = "<group>"; };
 		689EC1E2281B30D3008FEB58 /* FlutterSpellCheckPluginTest.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = FlutterSpellCheckPluginTest.mm; sourceTree = "<group>"; };
 		68B6091227F62F990036AC78 /* VsyncWaiterIosTest.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = VsyncWaiterIosTest.mm; sourceTree = "<group>"; };
-		73F12C22288F92FF00AFC3A6 /* FlutterViewControllerTest_mrc.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = FlutterViewControllerTest_mrc.mm; sourceTree = "<group>"; };
 		D2D361A52B234EAC0018964E /* FlutterMetalLayerTest.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = FlutterMetalLayerTest.mm; sourceTree = "<group>"; };
 		F7521D7226BB671E005F15C5 /* libios_test_flutter.dylib */ = {isa = PBXFileReference; lastKnownFileType = "compiled.mach-o.dylib"; name = libios_test_flutter.dylib; path = "../../../../out/$(FLUTTER_ENGINE)/libios_test_flutter.dylib"; sourceTree = "<group>"; };
 		F7521D7526BB673E005F15C5 /* libocmock_shared.dylib */ = {isa = PBXFileReference; lastKnownFileType = "compiled.mach-o.dylib"; name = libocmock_shared.dylib; path = "../../../../out/$(FLUTTER_ENGINE)/libocmock_shared.dylib"; sourceTree = "<group>"; };
@@ -109,12 +107,10 @@
 				0AC232F724BA71D300A85907 /* FlutterEngineTest.mm */,
 				0AC2330324BA71D300A85907 /* accessibility_bridge_test.mm */,
 				0AC2330B24BA71D300A85907 /* FlutterTextInputPluginTest.mm */,
-				0AC2330F24BA71D300A85907 /* FlutterBinaryMessengerRelayTest.mm */,
 				0AC2331024BA71D300A85907 /* connection_collection_test.mm */,
 				0AC2331224BA71D300A85907 /* FlutterEnginePlatformViewTest.mm */,
 				0AC2331924BA71D300A85907 /* FlutterPluginAppLifeCycleDelegateTest.mm */,
 				0AC2332124BA71D300A85907 /* FlutterViewControllerTest.mm */,
-				73F12C22288F92FF00AFC3A6 /* FlutterViewControllerTest_mrc.mm */,
 				D2D361A52B234EAC0018964E /* FlutterMetalLayerTest.mm */,
 			);
 			name = Source;

testing/ios/IosUnitTests/Tests/FlutterEngineConfig.xcconfig

@@ -1,3 +1,3 @@
 FLUTTER_ENGINE[arch=x86_64]=ios_debug_sim_unopt
 FLUTTER_ENGINE[arch=arm64]=ios_debug_sim_unopt_arm64
-FLUTTER_ENGINE=ios_debug_sim_unopt
+FLUTTER_ENGINE=ios_debug_sim_unopt_arm64