Support accessibility back gesture to pop route from view controller

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 != nil) {
+    [self popRoute];
+    return YES;
+  }
+  return NO;
+}
+
 + (BOOL)accessibilityIsOnOffSwitchLabelsEnabled {
   if (@available(iOS 13, *)) {
     return UIAccessibilityIsOnOffSwitchLabelsEnabled();

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

@@ -1365,6 +1365,20 @@ - (void)testItReportsAccessibilityOnOffSwitchLabelsFlagSet {
   XCTAssert((flags & (int32_t)flutter::AccessibilityFeatureFlag::kOnOffSwitchLabels) != 0);
 }
 
+- (void)testAccessibilityPerformEscapePopsRoute {
+  FlutterEnginePartialMock* mockEngine = [[FlutterEnginePartialMock alloc] init];
+  id mockNavigationChannel = OCMClassMock([FlutterBasicMessageChannel class]);
+  OCMStub([mockEngine navigationChannel]).andReturn(mockNavigationChannel);
+
+  FlutterViewController* viewController =
+      [[FlutterViewController alloc] initWithEngine:self.mockEngine nibName:nil bundle:nil];
+  [viewController accessibilityPerformEscape];
+
+  OCMVerify([mockNavigationChannel invokeMethod:@"popRoute" arguments:nil]);
+
+  [mockNavigationChannel stopMocking];
+}
+
 - (void)testPerformOrientationUpdateForcesOrientationChange {
   [self orientationTestWithOrientationUpdate:UIInterfaceOrientationMaskPortrait
                           currentOrientation:UIInterfaceOrientationLandscapeLeft