Finish plumbing message responses on method channels (#11877)

shell/platform/common/cpp/client_wrapper/include/flutter/method_channel.h

@@ -48,11 +48,18 @@ class MethodChannel {
     MethodCall<T> method_call(method, std::move(arguments));
     std::unique_ptr<std::vector<uint8_t>> message =
         codec_->EncodeMethodCall(method_call);
-    messenger_->Send(name_, message->data(), message->size());
+    messenger_->Send(name_, message->data(), message->size(), nullptr);
   }
 
-  // TODO: Add support for a version of InvokeMethod expecting a reply once
-  // https://github.com/flutter/flutter/issues/18852 is fixed.
+  // Sends a message to the Flutter engine on this channel expecting a reply.
+  void InvokeMethod(const std::string& method,
+                    std::unique_ptr<T> arguments,
+                    flutter::BinaryReply reply) {
+    MethodCall<T> method_call(method, std::move(arguments));
+    std::unique_ptr<std::vector<uint8_t>> message =
+        codec_->EncodeMethodCall(method_call);
+    messenger_->Send(name_, message->data(), message->size(), reply);
+  }
 
   // Registers a handler that should be called any time a method call is
   // received on this channel.

shell/platform/common/cpp/client_wrapper/plugin_registrar.cc

@@ -98,8 +98,8 @@ void BinaryMessengerImpl::Send(const std::string& channel,
                                const size_t message_size,
                                BinaryReply reply) const {
   if (reply == nullptr) {
-    std::cerr << "Calling BinaryMessengerImpl::Send expecting a reply, but the "
-                 "callback is null.\n";
+    FlutterDesktopMessengerSend(messenger_, channel.c_str(), message,
+                                message_size);
     return;
   }
   struct Captures {

shell/platform/darwin/common/framework/Headers/FlutterChannels.h

@@ -234,11 +234,7 @@ FLUTTER_EXPORT
  */
 - (void)invokeMethod:(NSString*)method
            arguments:(id _Nullable)arguments
-              result:(FlutterResult _Nullable)callback
-    // TODO: Add macOS support for replies once
-    // https://github.com/flutter/flutter/issues/18852 is fixed.
-    API_UNAVAILABLE(macos);
-
+              result:(FlutterResult _Nullable)callback;
 /**
  * Registers a handler for method calls from the Flutter side.
  *