Actually return a SemanticsServer service.

packages/flutter/lib/src/rendering/binding.dart

@@ -68,7 +68,8 @@ abstract class Renderer extends Object with Scheduler, MojoShell
   void initSemantics() {
     SemanticsNode.onSemanticsEnabled = renderView.scheduleInitialSemantics;
     provideService(mojom.SemanticsServer.serviceName, (core.MojoMessagePipeEndpoint endpoint) {
-      return new SemanticsServer();
+      mojom.SemanticsServerStub server = new mojom.SemanticsServerStub.fromEndpoint(endpoint);
+      server.impl = new SemanticsServer();
     });
   }
 

packages/flutter/lib/src/rendering/semantics.dart

@@ -439,8 +439,8 @@ class SemanticsNode extends AbstractNode {
 }
 
 class SemanticsServer extends mojom.SemanticsServer {
-  void addSemanticsListener(mojom.SemanticsListener listener) {
-    SemanticsNode.addListener(listener);
+  void addSemanticsListener(mojom.SemanticsListenerProxy listener) {
+    SemanticsNode.addListener(listener.ptr);
   }
   void tap(int nodeID) {
     SemanticsNode.getSemanticActionHandlerForId(nodeID, neededFlag: _SemanticFlags.canBeTapped)?.handleSemanticTap();