[engine] add back opt out for merged thread mode.

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

@@ -218,6 +218,12 @@ static BOOL DoesHardwareSupportWideGamut() {
     settings.leak_vm = leakDartVM.boolValue;
   }
 
+  NSNumber* enableMergedPlatformUIThread =
+      [mainBundle objectForInfoDictionaryKey:@"FLTEnableMergedPlatformUIThread"];
+  if (enableMergedPlatformUIThread != nil) {
+    settings.merged_platform_ui_thread = enableMergedPlatformUIThread.boolValue;
+  }
+
 #if FLUTTER_RUNTIME_MODE == FLUTTER_RUNTIME_MODE_DEBUG
   // There are no ownership concerns here as all mappings are owned by the
   // embedder and not the engine.

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

@@ -706,7 +706,7 @@ + (NSString*)generateThreadLabel:(NSString*)labelPrefix {
   fml::MessageLoop::EnsureInitializedForCurrentThread();
 
   uint32_t threadHostType = flutter::ThreadHost::Type::kRaster | flutter::ThreadHost::Type::kIo;
-  if (!settings.enable_impeller) {
+  if (!settings.enable_impeller || !settings.merged_platform_ui_thread) {
     threadHostType |= flutter::ThreadHost::Type::kUi;
   }
 
@@ -795,7 +795,7 @@ - (BOOL)createShell:(NSString*)entrypoint
       [](flutter::Shell& shell) { return std::make_unique<flutter::Rasterizer>(shell); };
 
   fml::RefPtr<fml::TaskRunner> ui_runner;
-  if (settings.enable_impeller) {
+  if (settings.enable_impeller && settings.merged_platform_ui_thread) {
     ui_runner = fml::MessageLoop::GetCurrent().GetTaskRunner();
   } else {
     ui_runner = _threadHost->ui_thread->GetTaskRunner();

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

@@ -472,16 +472,17 @@ - (void)testCanMergePlatformAndUIThread {
 #endif  // defined(TARGET_IPHONE_SIMULATOR) && TARGET_IPHONE_SIMULATOR
 }
 
-- (void)testCanNotUnMergePlatformAndUIThread {
+- (void)testCanUnMergePlatformAndUIThread {
 #if defined(TARGET_IPHONE_SIMULATOR) && TARGET_IPHONE_SIMULATOR
   auto settings = FLTDefaultSettingsForBundle();
   settings.enable_impeller = true;
+  settings.merged_platform_ui_thread = false;
   FlutterDartProject* project = [[FlutterDartProject alloc] initWithSettings:settings];
   FlutterEngine* engine = [[FlutterEngine alloc] initWithName:@"foobar" project:project];
   [engine run];
 
-  XCTAssertEqual(engine.shell.GetTaskRunners().GetUITaskRunner(),
-                 engine.shell.GetTaskRunners().GetPlatformTaskRunner());
+  XCTAssertNotEqual(engine.shell.GetTaskRunners().GetUITaskRunner(),
+                    engine.shell.GetTaskRunners().GetPlatformTaskRunner());
 #endif  // defined(TARGET_IPHONE_SIMULATOR) && TARGET_IPHONE_SIMULATOR
 }