Made sure to call the plugin registrant when registering a background isolate (flutter/engine#36383)

engine/src/flutter/lib/ui/platform_dispatcher.dart

@@ -586,6 +586,7 @@ class PlatformDispatcher {
   /// [token]. This is required if platform channels are to be used on a
   /// background isolate.
   void registerBackgroundIsolate(RootIsolateToken token) {
+    DartPluginRegistrant.ensureInitialized();
     __registerBackgroundIsolate(token._token);
   }
   @FfiNative<Void Function(Int64)>('PlatformConfigurationNativeApi::RegisterBackgroundIsolate')

engine/src/flutter/runtime/dart_plugin_registrant_unittests.cc

@@ -195,6 +195,54 @@ TEST_F(DartIsolateTest, DartPluginRegistrantNotFromBackgroundIsolate) {
       "_PluginRegistrant.register() was not called on background isolate");
 }
 
+TEST_F(DartIsolateTest, DartPluginRegistrantWhenRegisteringBackgroundIsolate) {
+  ASSERT_FALSE(DartVMRef::IsInstanceRunning());
+
+  std::vector<std::string> messages;
+  fml::AutoResetWaitableEvent latch;
+
+  AddNativeCallback(
+      "PassMessage",
+      CREATE_NATIVE_ENTRY(([&latch, &messages](Dart_NativeArguments args) {
+        auto message = tonic::DartConverter<std::string>::FromDart(
+            Dart_GetNativeArgument(args, 0));
+        messages.push_back(message);
+        latch.Signal();
+      })));
+
+  auto settings = CreateSettingsForFixture();
+  auto did_throw_exception = false;
+  settings.unhandled_exception_callback = [&](const std::string& error,
+                                              const std::string& stack_trace) {
+    did_throw_exception = true;
+    return true;
+  };
+
+  auto vm_ref = DartVMRef::Create(settings);
+  auto thread = CreateNewThread();
+  TaskRunners task_runners(GetCurrentTestName(),  //
+                           thread,                //
+                           thread,                //
+                           thread,                //
+                           thread                 //
+  );
+
+  auto kernel_path =
+      fml::paths::JoinPaths({GetFixturesPath(), kKernelFileName});
+  auto isolate = RunDartCodeInIsolate(
+      vm_ref, settings, task_runners,
+      "registerBackgroundIsolateCallsDartPluginRegistrant", {}, kernel_path);
+
+  ASSERT_TRUE(isolate);
+  ASSERT_EQ(isolate->get()->GetPhase(), DartIsolate::Phase::Running);
+
+  latch.Wait();
+
+  ASSERT_EQ(messages.size(), 1u);
+  ASSERT_EQ(messages[0],
+            "_PluginRegistrant.register() was called on background isolate");
+}
+
 }  // namespace testing
 }  // namespace flutter
 

engine/src/flutter/runtime/fixtures/dart_tool/flutter_build/dart_plugin_registrant.dart

@@ -41,6 +41,13 @@ void dartPluginRegistrantIsolate(SendPort sendPort) {
   sendPort.send(didCallRegistrantBeforeEntrypoint);
 }
 
+void registerBackgroundIsolate(List args) {
+  SendPort sendPort = args[0] as SendPort;
+  RootIsolateToken token = args[1] as RootIsolateToken;
+  PlatformDispatcher.instance.registerBackgroundIsolate(token);
+  sendPort.send(didCallRegistrantBeforeEntrypoint);
+}
+
 @pragma('vm:entry-point')
 void callDartPluginRegistrantFromBackgroundIsolate() async {
   ReceivePort receivePort = ReceivePort();
@@ -70,3 +77,16 @@ void dontCallDartPluginRegistrantFromBackgroundIsolate() async {
   }
   isolate.kill();
 }
+
+@pragma('vm:entry-point')
+void registerBackgroundIsolateCallsDartPluginRegistrant() async {
+  ReceivePort receivePort = ReceivePort();
+  Isolate isolate = await Isolate.spawn(registerBackgroundIsolate, [receivePort.sendPort, RootIsolateToken.instance]);
+  bool didCallEntrypoint = await receivePort.first;
+  if (didCallEntrypoint) {
+    passMessage('_PluginRegistrant.register() was called on background isolate');
+  } else {
+    passMessage('_PluginRegistrant.register() was not called on background isolate');
+  }
+  isolate.kill();
+}