[Impeller] Run simulator tests with Impeller enabled.

The Info.plist override was missed earlier.

The main changes to the tests are due to the UI thread merge with the
platform thread. The amendment to the "spawn" API are due to the fact
that the assertions checked for the presence of GrDirectContext which
will never exist with Impeller.

Unblocks #56706

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

@@ -270,12 +270,13 @@ - (void)testSetHandlerAfterRun {
     fml::AutoResetWaitableEvent latch;
     [engine run];
     flutter::Shell& shell = engine.shell;
-    engine.shell.GetTaskRunners().GetUITaskRunner()->PostTask([&latch, &shell] {
-      flutter::Engine::Delegate& delegate = shell;
-      auto message = std::make_unique<flutter::PlatformMessage>("foo", nullptr);
-      delegate.OnEngineHandlePlatformMessage(std::move(message));
-      latch.Signal();
-    });
+    fml::TaskRunner::RunNowOrPostTask(
+        engine.shell.GetTaskRunners().GetUITaskRunner(), [&latch, &shell] {
+          flutter::Engine::Delegate& delegate = shell;
+          auto message = std::make_unique<flutter::PlatformMessage>("foo", nullptr);
+          delegate.OnEngineHandlePlatformMessage(std::move(message));
+          latch.Signal();
+        });
     latch.Wait();
     [registrar.messenger setMessageHandlerOnChannel:@"foo"
                                binaryMessageHandler:^(NSData* message, FlutterBinaryReply reply) {
@@ -287,14 +288,11 @@ - (void)testSetHandlerAfterRun {
 
 - (void)testThreadPrioritySetCorrectly {
   XCTestExpectation* prioritiesSet = [self expectationWithDescription:@"prioritiesSet"];
-  prioritiesSet.expectedFulfillmentCount = 3;
+  prioritiesSet.expectedFulfillmentCount = 2;
 
   IMP mockSetThreadPriority =
       imp_implementationWithBlock(^(NSThread* thread, double threadPriority) {
-        if ([thread.name hasSuffix:@".ui"]) {
-          XCTAssertEqual(threadPriority, 1.0);
-          [prioritiesSet fulfill];
-        } else if ([thread.name hasSuffix:@".raster"]) {
+        if ([thread.name hasSuffix:@".raster"]) {
           XCTAssertEqual(threadPriority, 1.0);
           [prioritiesSet fulfill];
         } else if ([thread.name hasSuffix:@".io"]) {
@@ -429,10 +427,6 @@ - (void)testSpawnsShareGpuContext {
   std::shared_ptr<flutter::IOSContext> engine_context = [engine iosPlatformView]->GetIosContext();
   std::shared_ptr<flutter::IOSContext> spawn_context = [spawn iosPlatformView]->GetIosContext();
   XCTAssertEqual(engine_context, spawn_context);
-  // If this assert fails it means we may be using the software.  For software rendering, this is
-  // expected to be nullptr.
-  XCTAssertTrue(engine_context->GetMainContext() != nullptr);
-  XCTAssertEqual(engine_context->GetMainContext(), spawn_context->GetMainContext());
 }
 
 - (void)testEnableSemanticsWhenFlutterViewAccessibilityDidCall {
@@ -445,7 +439,6 @@ - (void)testEnableSemanticsWhenFlutterViewAccessibilityDidCall {
 - (void)testCanMergePlatformAndUIThread {
 #if defined(TARGET_IPHONE_SIMULATOR) && TARGET_IPHONE_SIMULATOR
   auto settings = FLTDefaultSettingsForBundle();
-  settings.enable_impeller = true;
   FlutterDartProject* project = [[FlutterDartProject alloc] initWithSettings:settings];
   FlutterEngine* engine = [[FlutterEngine alloc] initWithName:@"foobar" project:project];
   [engine run];
@@ -458,7 +451,6 @@ - (void)testCanMergePlatformAndUIThread {
 - (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];

shell/platform/darwin/ios/ios_context.mm

@@ -56,6 +56,7 @@
 }
 
 IOSRenderingBackend IOSContext::GetBackend() const {
+  // Overridden by Impeller subclasses.
   return IOSRenderingBackend::kSkia;
 }
 

testing/ios/IosUnitTests/App/Info.plist

@@ -46,8 +46,6 @@
 	</dict>
 	<key>FLTLeakDartVM</key>
 	<false/>
-	<key>FLTEnableImpeller</key>
-	<false/>
 	<key>FLTTraceSystrace</key>
 	<false/>
 	<key>FLTEnableDartProfiling</key>